Patent Publication Number: US-8971769-B2

Title: Development device including a removable seal to seal a supplied-developer and/or a collected-developer communicating area

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of and claims the benefit of priority from U.S. application Ser. No. 14/030,590, filed Sep. 18, 2013, which is a continuation application of U.S. application Ser. No. 12/700,834, filed Feb. 5, 2010, now U.S. Pat. No. 8,571,449, which is based upon and claims the benefit of priority from Japanese Patent Application Nos. 2009-025834, filed on Feb. 6, 2009, and 2009-298609, filed on Dec. 28, 2009, in the Japan Patent Office, the contents of each of which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a development device, a process cartridge, and an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction machine capable of at least two of these functions that includes the development device. 
     2. Discussion of the Background Art 
     In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices including at least two of those functions, etc., include a latent image carrier on which an electrostatic latent image is formed and a development device to develop the latent image with developer. 
     There are two types of developer used in electrophotographic images forming apparatuses, namely, one-component developer consisting of magnetic or non-magnetic toner and two-component developer including toner and carrier particles. Recently, two-component developer has come to be widely used because its durability and image quality are better than those of one-component developer. Development devices using two-component developer (hereinafter “two-component development devices”) typically include a rotary cylindrical developer carrier (e.g., development sleeve) inside which a stationary magnetic field generator having multiple magnetic poles is provided to carry the developer on the development sleeve. 
     In certain known development devices, the magnetic field generator has five magnetic poles that can generate magnetic fields of sufficient strength for the development sleeve to carry the developer. The five magnetic poles include an attraction pole, a pre-development transport pole, a development pole, a release pole, and a post-development transport pole. The developer is attracted to a circumferential surface of the development sleeve at a position corresponding to the attraction pole (hereinafter “attraction portion”), and the pre-development pole generates a magnetic field for the development sleeve to transport the developer carried on the development sleeve to a development area or range facing the latent image carrier. The development pole contributes to latent image development in the development range, and the release pole contributes to separating the developer that has passed through the development range from the development sleeve. In this known configuration, the post-development transport pole is disposed between the development pole and the release pole and generates a magnetic field for the development sleeve to reliably transport the developer that has passed through the development range to the release position. In addition, in this known configuration, a developer regulator (e.g., doctor blade) is disposed facing the development sleeve between the attraction pole and the pre-development pole to adjust the amount of the developer carried to the development range. 
     With this configuration, processes of attracting the developer to the circumferential surface of the development sleeve, transporting the developer to the development range, developing the latent image with the developer, and releasing the developer from the development sleeve can be performed reliably. Alternatively, in certain known development devices, the magnetic field generator further includes a developer regulation pole disposed between the attraction pole and the pre-development pole, facing the developer regulator, and does not include the post-development transport pole. 
     At present, it is preferred that the development devices be more compact due to an increasing demand for more compact image forming apparatuses. The development devices can be more compact by using a development sleeve of reduced diameter. 
     However, in the known development devices, there in a practical limit to how much the diameter of the development sleeve can be reduced because it becomes difficult to reliably attract the developer to the development sleeve, transport the developer to the development range, develop the latent image with the developer, and release the developer from the development sleeve. Although magnets capable of generating a magnetic field of sufficient intensity are required to perform these processes reliably, the size of magnets increases as the intensity increases and therefore, it is difficult to reduce the diameter of the development sleeve inside which such large magnets are provided. 
     In view of the foregoing, there is a need to reduce the diameter of the developer carrier to make the development devices more compact while performing the above-described processes reliably, which the known image forming apparatuses fail to do. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, one illustrative embodiment of the present invention provides a development device. 
     The development device includes a developer containing part containing two-component developer including toner and magnetic carrier particles, a cylindrical developer carrier to carry by rotation the developer supplied from the developer containing part to a development range where the developer carrier faces an image carrier, a first developer transport member disposed in the developer containing part, to supply the developer to the developer carrier while transporting the developer in an axial direction of the developer carrier, and a magnetic field generator disposed inside the developer carrier, having three developer-carrying magnetic poles each capable of generating a magnetic field to keep the developer on a circumferential surface of the developer carrier. 
     The three developer-carrying magnetic poles consist of a development pole to generate a first magnetic field in the development range, a pre-development pole to generate a second magnetic field to transport the developer supplied from the developer containing part to the development range, and a post-development pole to generate a third magnetic field disposed between the first magnetic field and the second magnetic field, to transport the developer that has passed the development range to a release position where the developer is separated from the circumferential surface of the developer carrier. The second magnetic field causes the developer supplied from the developer containing part to be attracted to the circumferential surface of the developer carrier at a developer attraction position. The first magnetic field and the second magnetic field together keep the developer on the circumferential surface of the developer carrier from the developer attraction position to the development range. The first magnetic field and the third magnetic field together keep the developer on the circumferential surface of the developer carrier from the development range to the release position. 
     Another illustrative embodiment of the present invention provides a process cartridge that is removably installable to an image forming apparatus. The process cartridge includes the development device described above and at least one of an image carrier on which a latent image is formed, a charging member disposed adjacent to the image carrier, to charge a surface of the image carrier, and a cleaning member to remove any toner remaining on the surface of the image carrier after the toner image is transferred from the image carrier. 
     Yet another illustrative embodiment of the present invention provides a image forming apparatus including an image carrier on which a latent image is formed, a charging member disposed adjacent to the image carrier, to charge a surface of the image carrier, the development device described above, a transfer member to transfer the toner image onto a sheet of recording media, and a cleaning member to remove any toner remaining on the surface of the image carrier after the toner image is transferred from the image carrier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  illustrates an example of a schematic configuration of an image forming apparatus according to an illustrative embodiment; 
         FIG. 2  is a schematic diagram illustrating a configuration of a development device according to an illustrative embodiment; 
         FIG. 3  illustrates a flow of developer in a developer containing part in the development device shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional diagram illustrating the development device shown in  FIG. 2 ; 
         FIG. 5  is a cross-sectional diagram illustrating an arrangement of magnets in a development roller in the development device shown in  FIG. 2 ; 
         FIG. 6  is a cross-sectional diagram illustrating an arrangement of magnets in a development roller in a comparative example; 
         FIG. 7  is a graph illustrating a distribution of magnetic flux density in normal direction in a three-pole configuration. 
         FIG. 8  illustrates the configuration of the development device shown in  FIG. 2  together with a waveform of the magnetic fields around the development roller; 
         FIG. 9A  is a graph illustrating a waveform of magnetic flux density in normal direction when an angle θ 3  is 180° in the development device shown in  FIG. 2 ; 
         FIG. 9B  illustrates movement of the developer calculated using the waveform shown  FIG. 9A ; 
         FIG. 10A  is a graph illustrating a waveform of magnetic flux density in normal direction when an angle θ 3  is 150° in a comparative example; 
         FIG. 10B  illustrates movement of the developer based on the result calculated using the waveform shown  FIG. 10A ; 
         FIG. 11  is a schematic diagram illustrating a configuration of a five-pole development device according to another comparative example; 
         FIG. 12  is a graph illustrating a comparison of the magnetic force in normal direction to the surface of the development sleeve around a release portion between in the three-pole development device shown in  FIG. 8  and the five-pole development device shown in  FIG. 11 ; 
         FIG. 13  is a graph illustrating magnetic attraction in normal direction at respective positions around the release portion when the peak of the magnetic flux density in normal direction in a pre-development pole N 2  is set to 30 mT and 60 mT; 
         FIG. 14  is a graph comparing the magnetic attraction in normal direction around the release portion measured when a relation Br 1 ≧Br 2  is satisfied and when this relation is not satisfied; 
         FIG. 15  is a graph illustrating distribution of the magnetic flux density in normal direction at respective positions on the surface of the development sleeve under three different conditions in a single-pole configuration; 
         FIG. 16  illustrates an example of distribution of magnetic flux density in normal direction around the development sleeve in which the density peak of the magnetic flux of the opposite polarity is considered; 
         FIG. 17  is a graph illustrating a relation between the release angle and the amount of carried-over developer; 
         FIG. 18A  is a graph illustrating the relation between the angle position on the circumferential surface of the development sleeve and magnetic force in the normal direction; 
         FIG. 18B  is a graph illustrating the relation between the angle position on the circumferential surface of the development sleeve and magnetic force in a tangent direction; 
         FIG. 19  is a graph illustrating the relation between the release angle and a range where the magnetic force in the tangent direction is zero; 
         FIG. 20  illustrates a relation between the vector of magnetic force and the distribution of the magnetic flux density in the normal direction around the development roller; 
         FIG. 21  is a schematic diagram illustrating a configuration of a development device according to another comparative example; 
         FIGS. 22A and 22B  are graphs respectively illustrating relations between toner concentration in weight percent and positions in the development device shown in  FIG. 2  and that in a development device according to a comparative example 4; 
         FIG. 23  is a graph illustrating deformation amount of development sleeves whose materials and diameters are different; 
         FIG. 24  is a graph illustrating torque of the development sleeve in the first embodiment and a comparative example 6; 
         FIG. 25  is a graph illustrating the load calculated based on the magnetic attraction when the magnetic flux density in the normal direction at a center of the pre-development pole is varied; 
         FIG. 26  is a schematic diagram illustrating a configuration of a development device according to a second embodiment; 
         FIG. 27  illustrates the configuration of the development device together with a waveform of the magnetic fields around a development roller in the second embodiment; 
         FIG. 28  is a graph illustrating a distribution of magnetic flux density in the normal direction in the development device shown in  FIG. 26 ; 
         FIG. 29  is a schematic diagram illustrating a configuration of a development device according to a third embodiment; 
         FIG. 30  is a schematic diagram illustrating a development device as a variation of the third embodiment; 
         FIG. 31  is a schematic diagram illustrating a configuration of a development device according to a fourth embodiment; 
         FIG. 32  illustrates the configuration of the development device together with a waveform of the magnetic fields around a development roller in the fourth embodiment; 
         FIG. 33  is a schematic diagram illustrating a configuration of a development device according to a fifth embodiment; 
         FIG. 34  is a schematic diagram illustrating a configuration of a development device according to a sixth embodiment; 
         FIG. 35  is a schematic diagram illustrating a configuration of a development device according to a seventh embodiment; 
         FIG. 36  is a schematic diagram illustrating a configuration of a variation of the development device shown in  FIG. 31 ; 
         FIGS. 37A and 37B  are graphs showing relations between the amount of developer passing through a regulation gap and the width of the regulation gap when the peak of the magnetic flux density in the normal direction in the pre-development pole N 2  is 15 mT and 30 mT, respectively; 
         FIG. 38  illustrates a flow and a distribution of the developer in developer transport paths in the development device according to an illustrative embodiment; 
         FIG. 39  illustrates a lead angle of a screw member; 
         FIG. 40  is a graph illustrating the relation between the lead angle and transport velocity of the screw member; 
         FIG. 41  illustrates a flow of the developer around a downstream end in a circulation path in a developer transport direction; 
         FIG. 42  illustrates a screw member having a smaller lead angle; 
         FIG. 43  illustrates a screw member having a larger lead angle; 
         FIG. 44  is a graph illustrating the relation between the lead angle of the screw member and the amount of developer transported upward in a bring-up portion; 
         FIG. 45  illustrates a cross section of a development device cut at the position of the bring-up portion in a ninth embodiment; 
         FIG. 46  illustrates a cross section of a development device cut at the position of a bring-up portion in a comparative example 7; 
         FIG. 47  schematically illustrates a downstream end portion in a circulation path in the development device shown in  FIG. 45  viewed from above; 
         FIG. 48  illustrates a triangular bring-up port; 
         FIG. 49  illustrates a trapeziform bring-up port; 
         FIG. 50  illustrates a rounded bring-up port; 
         FIG. 51  illustrates a flow and a distribution of developer in developer transport paths in an illustrative embodiment; 
         FIG. 52  is an overhead view illustrating a downstream end portion in a circulation path in a development device in which a bring-up port  41  is divided into two; 
         FIG. 53  is a side view illustrating the downstream end portion in the circulation path in the development device shown in  FIG. 52 ; 
         FIG. 54  is a graph illustrating the relation between the shape of the bring-up port and dispersion coefficient of toner; 
         FIG. 55  illustrates a flow of the developer in a given area in the circulation path in a developer transport direction; 
         FIG. 56  illustrates the developer transport path in short of developer; 
         FIG. 57  illustrates conditions to prevent shortage of developer in the developer transport path; 
         FIG. 58  is a graph illustrating the relation between the lead angle of the screw member and dispersibility of supplied toner; 
         FIG. 59  is a graph illustrating the relation between the opening area of the bring-up port and the amount of transported developer. 
         FIG. 60  illustrates a configuration to increase dispersibility of toner; 
         FIG. 61  illustrates a circulation screw provided with paddles; 
         FIG. 62  illustrates a configuration of the circulation screw in which a bladed spiral is partly cut off; 
         FIG. 63  illustrates another configuration of the circulation screw in which a bladed spiral is partly cut off; 
         FIG. 64  is a graph illustrating toner dispersibility of various circulation screws whose shapes are different; 
         FIG. 65  is a cross-sectional diagram illustrating an upstream end portion of the circulation path in the developer transport direction; 
         FIG. 66  is a graph illustrating the amount of developer at respective positions in the developer transport direction in the developer transport path when the transport velocity of the developer is constant across the entire developer transport path; 
         FIG. 67  is a graph illustrating the amount of developer at respective positions in the developer transport direction in the developer transport path when the transport velocity of the developer is varied depending on the position in the developer transport direction; 
         FIG. 68  illustrates a cross section of an image forming unit that is a process cartridge to which an eleventh embodiment is applicable; 
         FIG. 69  is a cross-sectional diagram illustrates a configuration of a development device including a paddle as an agitation member; 
         FIG. 70  is a perspective view of the paddle shown in  FIG. 69 ; 
         FIG. 71A  is a cross-sectional diagram illustrates a configuration of a development device including a roller member as an agitation member; 
         FIG. 71B  is a perspective view of the roller member shown in  FIG. 71A ; 
         FIG. 72A  is a cross-sectional diagram illustrates a configuration of a development device including a wire member as an agitation member; 
         FIG. 72B  is a perspective view of the wire member shown in  FIG. 72A ; 
         FIG. 73  schematically illustrates relative positions of a photoconductor, a development sleeve, and a development gear in an illustrative embodiment; 
         FIG. 74  schematically illustrates relative positions of a photoconductor, a development sleeve, and a development gear in a comparative example; 
         FIG. 75  illustrates flow of the developer on a cross section of the development device perpendicular to an axial direction; 
         FIG. 76  illustrates an N-N′ cross section of the development device shown in  FIG. 75 ; 
         FIG. 77A  is a cross sectional diagram illustrating a configuration in which two seam members respectively seal the supply path and the circulation path; 
         FIG. 77B  illustrates a configuration in which the two seal members are pulled out from the development device separately; 
         FIG. 77C  illustrate a configuration in which the two seal members are united as a single member to be pulled out simultaneously from the development device; 
         FIG. 78A  is a cross sectional diagram illustrating a configuration in which a single seam member seals both the supply path and the circulation path; 
         FIG. 78B  illustrates a configuration in which the seal member is pulled out from the development device horizontally; 
         FIG. 78C  illustrate a configuration in which the seal member is pulled out from the development device from above; 
         FIG. 79A  is a cross sectional diagram illustrating a configuration in which two seam members seal the supply path; 
         FIG. 79B  illustrates a configuration in which the two seal members are pulled out from the development device separately; 
         FIG. 79C  illustrate a configuration in which the two seal members are united as a single member to be pulled out simultaneously from the development device; 
         FIG. 80A  is a cross sectional diagram illustrating a configuration in which a single seam member seals the supply path; 
         FIG. 80B  illustrates a configuration in which the seal member is pulled out from the development device horizontally; 
         FIG. 80C  illustrate a configuration in which the seal member is pulled out from the development device from above; 
         FIG. 81A  is a cross sectional diagram illustrating a configuration in which two seam members seal the circulation path; 
         FIG. 81B  illustrates a configuration in which the two seal members are pulled out from the development device separately; 
         FIG. 81C  illustrate a configuration in which the two seal members are united as a single member to be pulled out simultaneously from the development device; 
         FIG. 82A  is a cross sectional diagram illustrating a configuration in which a single seam member seals the circulation path; 
         FIG. 82B  illustrates a configuration in which the seal member is pulled out from the development device horizontally; and 
         FIG. 82C  illustrate a configuration in which the seal member is pulled out from the development device from above. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views thereof, and particularly to  FIG. 1 , a color image forming apparatus according to an illustrative embodiment of the present invention is described. 
       FIG. 1  is a schematic diagram illustrates a configuration of an image forming apparatus  100  that in the present embodiment is a printer (hereinafter “printer  100 ”). The printer  100  is a tandem multicolor image forming apparatus and includes four image forming units  17 K,  17 M,  17 Y, and  17 C for forming yellow (Y), cyan (C), magenta (M), and black (K) single-color toner images, respectively. It is to be noted that the subscripts Y, M, C, and K attached to the end of each reference numeral indicate only that components indicated thereby are used for forming Y, M, C, and K images, respectively, and hereinafter may be omitted when color discrimination is not necessary. 
     An endless transfer-transport belt  15  wound around support rollers  18  and  19  is provided beneath the image forming units  17 . The support rollers  18  and  19  are respectively disposed on a downstream side and an upstream side in the belt transport direction. An upper side of the transfer-transport belt  15  rotates in a direction indicated by an arrow shown in  FIG. 1  (hereinafter “belt transport direction”) while carrying a sheet P (recording medium) thereon. Transfer bias rollers  5 K,  5 M,  5 Y,  5 C are provided facing the respective image forming units  17 K,  17 M,  17 Y, and  17 C via the transfer-transport belt  15 . 
     The printer  100  further includes a fixing device  24  disposed downstream from the downstream support roller  18  in the belt transport direction and a discharge tray  25  provided on an upper portion of a main body of the printer  100 . The fixing device  24  fixes a toner image on the sheet P after the sheet P is separated from the transfer-transport belt  15 , after which the sheet P is discharged onto the discharge tray  25 . 
     The printer  100  further includes sheets cassettes  20 ,  21 , and  22  each containing multiple sheets P, a feed unit  26  to feed the sheets P from the sheets cassettes  20 ,  21 , and  22  to the image forming units  17 , and a pair of registration rollers  23 . The registration rollers  23  forward the sheet P sent from the sheet cassettes  20  through  22  to a transfer positions where the transfer-transport belt  15  faces the respective image forming units  17 . 
     It is to be noted that in the configuration shown in  FIG. 1 , the transfer-transport belt  15  is disposed obliquely to reduce the width, that is, the length of the printer  100  in the lateral direction in  FIG. 1 , and accordingly the belt transport direction indicated by the arrow is oblique. With this configuration, the width of the printer  100  can be only a length slightly greater than the length of A3 sheets in their longitudinal direction. In other words, the width of the printer  100  can be significantly reduced to a length only necessary to contain the sheets. 
     Each image forming unit  17  includes a drum-shaped photoconductor  1  serving as a latent image carrier. A charger  2  serving as a charging member to charge a surface of the photoreceptor  1 , a developing device  3  to develop an electrostatic latent image formed on the photoconductor  1 , and a cleaner  6  to clean the surface of the photoconductor  1  are provided around the photoreceptor  1 . An exposure unit  16  directs writing light (e.g., writing beam) L onto the surface of each photoconductor  1  between the charger  2  and the development device  3 . Thus, each image forming unit  17  has a known configuration. The photoconductor  1  may be a belt instead of a drum. 
     In the present embodiment, at least the developing device  3  and the photoconductor  1  are integrated into a single process cartridge that is removably installed in the body of the image forming apparatus  100 . 
     In the above-described printer  100 , when users instructs the printer to start image formation, each image forming unit  17  stars to form a single color toner image. More specifically, in each image forming unit  17 , the photoconductor  1  is rotated by a main motor, not shown, and is charged uniformly at a portion facing the charger  2  as the charging process. Then, the exposure unit  16  directs writing beams L onto the respective photoconductors  1  according to yellow, cyan, magenta, and black image data decomposed from multicolor image data, thus forming electrostatic latent images thereon. Each latent image is then developed by the development device  3 , and thus single-color toner images are formed on the respective photoconductors  1 . While the processes described above are performed, the sheets P are fed one by one from one of the sheet cassettes  20  through  22  by the feed unit  26  to the registration rollers  23 , which forward the sheet P to the transfer-transport belt  15 , timed to coincide with the arrival of the toner images formed on the respective photoconductors  1 . Then, the transfer-transport belt  15  transports the sheet P to the respective transfer positions. 
     When the surface of each photoconductor drum  1  carrying the toner image reaches a portion facing the transfer bias roller  5  via the transfer-transport belt  15 , the toner image is transferred by the bias applied by the transfer bias roller  5  from the photoconductor  5  onto the transfer-transport belt  15 . Thus, the K, M, Y, and C toner images are sequentially transferred from the respective photoconductors  1  and superimposed one on another on the sheet P, forming a multicolor toner image on the sheet P. The sheet P on which the multicolor toner image is formed is then separated from the transfer-transport belt  15 , and then the fixing device  54  fixes the image on the sheet P thereon, after which the sheet P is discharged onto the discharge tray  25 . 
     After the toner image is transferred from each photoconductor  1 , the cleaner  6  removes any toner remaining thereon, and a discharge lamp, not shown, removes electrical potentials remaining on the photoconductor  1  as required. Then, the charger  2  again charges the surface of the photoconductor  1 . 
     Descriptions are given below of the development devices  3 K,  3 M,  3 Y, and  3 C according to a first embodiment, which have a similar configuration except that the color of the toner used therein is different. 
     First Embodiment 
       FIG. 2  illustrates a schematic configuration of the development device  3  according to a first embodiment of the present invention. The development device  3  is disposed facing the photoconductor  1  that rotates clockwise, that is, in a direction indicated by arrow a in  FIG. 2 . A casing  33  of the development device  3  contains two-component powder developer  32  including magnetic carrier particles and organic or inorganic toner particles. The development device  3  includes a development roller  34  formed with a development sleeve  34   a  and a magnet roller  34   b  disposed inside the development sleeve  34   a . The development sleeve  34   a , serving as a developer carrier, carries the developer  32  supplied from the casing  33  on a circumferential surface thereof and transports by rotation the developer to a development range A. In the development range A, the developer  32  is supplied to the latent image formed on the photoconductor  1 , thus developing the latent image into a toner image. The magnet roller  34   b  includes multiple magnets whose positions are fixed relative to the development device  3 . The development device  3  further includes a developer regulator  35  to adjust the amount (e.g., layer thickness) of the developer  32  carried on the development sleeve  34   a.    
     It is to be noted that reference characters  34   p  represents a center of rotation of the development sleeve  34   a ,  46  represents a release portion where the developer  32  leaves the circumferential surface of the development sleeve  34   a , and  47  represents a attraction portion (developer attraction position) where the developer  32  is carried onto the development sleeve  34   a  from a supply path  37 . 
     The development device  3  further includes two developer transport members, namely, a supply screw  39  and a circulation screw  40 , both disposed in substantially parallel to a roller shaft  34   c  shown in  FIG. 5  (axial direction) of the development roller  34 . Each of the supply screw  39  and the circulation screw  40  includes a shaft and a bladed spiral provided on the shaft and transports the developer unidirectionally along the shaft (hereinafter “developer transport direction”) while rotating. Thus, the development device  3  is unidirectional circulation type that transports the developer unidirectionally. An inner wall of the development casing  33  as well as a partition  36  divide the space inside the casing  33  into the supply path  37  in which the supply screw  39  is disposed and a circulation path  38  in which the circulation screw  40  is disposed that are arranged vertically via the partition  36 . A port  41  (first communication port) and a port  42  (second communication port), both shown in  FIG. 3 , are respectively formed in both end portions of the partition  36  in a direction perpendicular to the surface of the paper on which  FIG. 2  is drawn, and the supply path  37  and the circulation path  38  communicate each other through the ports  41  and  42 . 
     Additionally, an end portion of the partition  36  on the side of the development sleeve  34   a  stands vertically in  FIG. 2  to enclose the supply screw  39  and thus forms a barrier  43 . The barrier  43  and the inner wall of the casing  33  together form an opening that opens toward the development sleeve  34   a , and the developer  32  is supplied from the supply path  37  through this opening to the development sleeve  34   a . This opening extends in a longitudinal direction, that is, the axial direction, of the development roller  34  so that the developer  32  can be supplied to the development sleeve  34   a  across the entire width of an image range to be developed. 
     It is to be noted that, in the present embodiment, because the amount of the developer  32  in the supply path  37  decreases as the developer  32  flows downstream in the developer transport direction in the supply path  37 , the height of the barrier  43  decreases toward downstream in the developer transport direction. 
     As shown in  FIG. 2 , each of the supply path  32  and the circulation path  38  contain the developer  32 . The circulation screw  40  is in substantially parallel to the supply screw  39  and transports the developer  32  in a direction opposite the direction in which the supply screw  39  transports the developer  32 . The developer  32  is circulated in the casing  33  through the ports  41  and  42  (shown in  FIG. 3 ) formed in the both end portions of the partition  36  as the supply screw  39  and the circulation screw  40  rotate. In  FIG. 2 , the supply screw  39  rotates clockwise, and the circulation screw  40  rotates counterclockwise similarly to the development sleeve  34   a.    
     The developer  32  contained in the supply path  37  is supplied onto a circumferential surface of the development sleeve  34   a  while transported by the supply screw  39 . More specifically, the developer  32  overstrides the barrier  43  as the supply screw  39  rotates or is attracted by the magnetic force exerted by the magnetic roller  34   b  provided inside the development sleeve  34   a . The developer  32  sent from the supply path  37  is carried on the development roller  34   a  in the attraction portion  47 , attracted by the magnetic force exerted by the magnetic roller  34   b  and is transported in a direction indicate by arrow B as the development sleeve  34   a  rotates. While the developer  32  carried on the development sleeve  34   a  passes a portion facing the developer regulator  35  (hereinafter “developer regulation portion”), the developer regulator  35  scrapes off excessive developer  32  from the development sleeve  34   a  as indicated by arrow B 1 . Thus, only a predetermined or given amount of the developer  32  passes the portion facing the developer regulator  35  in the direction indicated by arrow B. 
     Then, the predetermined amount of the developer  32  passes through the development range A as indicated by arrow B 2 , after which the developer  32  leaves the development sleeve  34   a  and flows to a bottom portion  33   b  of the casing  33  and thus enters the circulation path  38 . Thus, the developer  32  that is not supplied to the photoconductor  1  but remains on the development sleeve  34   a  after passing through the development range A is collected in the circulation path  38  instead of being transported to the supply path  37  immediately as the development sleeve  34   a  rotates. In the circulation path  38 , the collected developer  32  is mixed with fresh toner supplied thereto and then again sent to the supply path  37 . Therefore, only sufficiently agitated developer  32  can be present in the supply path  37 . The developer that reaches a downstream end portion in the developer transport direction in the supply path  37  as well as the developer that has left the development sleeve  34   a  after passing the development range A are transported through the circulation path  38  and then sent to an upstream end portion of the supply path  37 . The developer  32  in the circulation path  38  includes the developer  32  whose toner concentration is decreased while it passes through the development range A. Therefore, fresh toner is supplied to the circulation path  38  according to toner consumption calculated based on data of latent images or a detected toner concentration in the circulation path  38 . Thus, the developer  32  having a proper toner concentration can be supplied to the supply path  37 . 
       FIG. 3  illustrates a flow of the developer  32  in the casing  33  viewed in the direction indicated by arrow C in  FIG. 2 .  FIG. 4  is a cross-sectional view illustrating the supply screw  39  and the circulation screw  40  viewed in the direction indicated by arrow C in  FIG. 2 . In  FIGS. 3 and 4 , arrows indicate the flow of the developer  32  in the development device  3 . 
     As shown in  FIGS. 2 ,  3 , and  4 , because the supply path  37  and the circulation path  38  are arranged vertically, the developer  32  flows down through the port  42  serving as the second communication port (hereinafter also “falling port  42 ”) disposed on the right in the drawings, connecting the downstream end portion of the supply path  37  to the upstream end portion of the circulation path  38  in the developer transport direction. By contrast, the developer  32  is brought up through the port  41  serving as the first communication port (hereinafter also “bring-up port  41 ”) disposed on the left in the drawings, connecting the downstream end portion of the circulation path  38  to the upstream end portion of the supply path  37  in the developer transport direction. The developer  32  is pumped up by the pressure of the developer  32  accumulated in the downstream end portion of the circulation path  38  through the bring-up port  41  to the supply path  37 . As shown in  FIGS. 3 and 4 , a toner supply port  45  through which fresh toner is supplied to the development device  3  is formed in an upper portion of the casing  33 . The toner supply port  45  correspond to the position of the falling port  42  formed in the partition  36 , and the toner supplied through it flows down therethrough to an upstream portion of the circulation path  38 . 
     Not all of the developer  32  sent from the circulation path  38  to the supply path  37  reaches the downstream end of the supply path  37  in the developer transport direction of the supply screw  39 . As indicated by arrow B shown in  FIG. 3 , a certain amount of the developer  32  is supplied to the development sleeve  34   a  in mid-course of transportation in the supply path  37 , passes through the development range A, and then collected in the circulation path  38 . Thus, the developer  32  can be supplied onto the circumferential surface of the development sleeve  34   a  across a substantially entire axial length of the development sleeve  34   a . Therefore, the amount of the developer  32  transported by the supply screw  39  in the supply path  37  decreases gradually as the developer  32  flows downstream in the supply path  37 . By contrast, as the developer  32  flows downstream in the circulation path  38 , the amount of the developer  32  transported by the circulation screw  40  in the circulation path  38  increases gradually and is not uniform in the circulation path  38 . 
     In the present embodiment, as described above, the developer leaves the development sleeve  34   a  after passes through the development range A and is collected in the circulation path  38 . The developer whose toner concentration is decreased is not immediately supplied to the supply path  37 , but the toner concentration thereof is adjusted in the circulation path  38 , and thus the toner concentration can be kept constant across the supply path  37 . 
     Development Roller 
     The development roller  34  is described in further detail below. 
     As shown in  FIG. 2 , the magnet roller  34   b  has three magnetic poles each generating a magnetic field sufficiently strong to keep the developer  32  on the circumferential surface of the development sleeve  34   a  (hereinafter “developer-carrying magnetic pole”), namely, two north poles N 1  and N 2 , and a south pole S 1 .  FIG. 5  is a cross-sectional diagram illustrating an arrangement of the magnets of the magnet roller  34   b  inside the development sleeve  34   a . In the first embodiment, the magnet forming the pole S 1  (hereinafter also “magnet S 1 ”) of the magnet roller  34   b  is 3 mm in height and 2 mm in width in its cross section. The magnets respectively forming the poles N 1  and N 2  (hereinafter also “magnets N 1  and N 2 ”) are 2 mm in height and 2 mm in width in there cross sections. 
     In the present embodiment, the maximum magnetic flux density of the north poles N 1  and N 2 , and the south pole S 1  in the normal direction to the development sleeve  34   a  is not less than 10 mT. When this maximum magnetic flux density is not less than 10 mT, the strength of the magnetic fields generated by these developer-carrying magnetic poles is sufficient for keeping the developer  32  on the circumferential surface of the development sleeve  34   a.    
     As shown in  FIG. 5 , the development roller  34  includes the roller shaft  34   c  disposed at the center of rotation  34   p  of the development sleeve  34   a , and the three magnets S 1 , N 1 , and N 2  are disposed around the roller shaft  34   c . In the present embodiment, when the diameter of the roller shaft  34   c  is 3 mm and the development sleeve  34   a  has a sleeve thickness of 0.5 mm and a diameter of 9 mm, these magnets can be provided inside the development sleeve  34   a . It is to be noted that the sides of the cross section of each magnet is preferably not less than 2 mm because accuracy in processing is lower if it is extremely short. 
       FIG. 6  illustrates, as an example, disposing five developer-carrying magnetic poles inside a development roller  34 Z (hereinafter also “five-pole configuration”). Sizes of the development sleeve  34   a Z and a roller shaft  34   c Z are identical to those in the development roller  34  shown in  FIG. 5 . A magnet roller  34   b Z includes a magnet  34   s  forming a development pole, a magnet  34   r  forming a attraction pole, a magnet  34   u  forming a release pole, and other two magnets. When the magnet  34   s  has the identical size to that of the magnet S 2  shown in  FIG. 5 , and the other four magnets have the identical size to that of the magnets N 1  and N 2  shown in  FIG. 5 , the magnets overlap as shown in  FIG. 6 , and thus the five magnets cannot be provided inside the development sleeve  34   a Z. 
     Therefore, the present embodiment uses the development roller  34   b  that is a “three-pole development roller” inside which three magnets are provided (hereinafter “three-pole configuration”). Thus, by reducing the number of the magnets provided in the magnet roller  34   b , the space inside the development sleeve  34   a  necessary for the magnet roller  34   b  can be reduced, that is, the diameter of the development sleeve  34   a  can be reduced. When the diameter of the development sleeve  34   a  is the same, the space for each magnet forming a single developer-carrying magnetic pole can be increased in the three-pole configuration from that in the five-pole configuration. Therefore, even when the diameter of the development sleeve  34   a  is so small that the each magnet in the five-pole configuration cannot generate a magnetic field of sufficient strength, each magnet in the three-pole configuration can generate a magnetic field of sufficient strength. 
     It is to be noted that the magnetic field generator is not limited to the magnet roller  34   b  in which magnets are embedded. Alternatively, magnetic poles similar to those generated by the magnet roller  34   b  can be generated by forming a cylindrical member with a mixture of resin and magnetic powder, and disposing a magnetizing yoke around the cylindrical member to magnetize it. Although the space for the magnetic yoke inside the development sleeve is limited when the diameter of the development sleeve is smaller, the space can be larger in the three-pole configuration. 
     Thus, in the present embodiment, by reducing the diameter of the development sleeve  34   a , the size of the development device  3  can be reduced, and accordingly the process cartridge (image forming unit  17 ) including development device  3  can be more compact. Further, the image forming apparatus  100  that includes multiple process cartridges can be more compact. 
     Release of Developer from the Development Sleeve 
     Next, release of the developer  32  from the development sleeve  34   a  (hereinafter simply “release of developer”) in the first embodiment is described below. 
     As shown in  FIG. 2 , the pole S 1  is disposed facing the photoconductor  1  and serves as a development pole (hereinafter also “development pole S 1 ”) to generate a first magnetic field in the development range A. 
     The pole N 2  is disposed upstream from the development pole S 1  in the rotational direction of the development sleeve  34   a  and functions as both the attraction pole and the pre-development transport pole (hereinafter also “pre-development pole N 2 ”). The pre-development pole N 2  generates a second magnetic field to cause the developer supplied from the developer containing part (supply path  37 ) to be attracted to the circumferential surface of the development sleeve  34   a . The pre-development pole N 2  also serves as a developer regulation pole that generates a magnetic field in a developer regulation area where the development sleeve  34   a  faces the developer regulator  35 . 
     The developer is kept on the circumferential surface of the development sleeve  34   a  (hereinafter simply “surface of the development sleeve  34   a ”) by the second magnetic field generated by the pre-development pole N 2  and the first magnetic field development pole S 1  from the attraction portion  47  to the development range A. 
     The pole N 1  is disposed downstream from the development pole S 1  in the rotational direction of the development sleeve  34   a  and functions as both the post-development transport pole and the release pole (hereinafter also “post-development pole N 1 ”) to generate a third magnetic field disposed between the first magnetic field and the second magnetic field, to transport the developer that has passed the development range A to the release portion  46  (release position) where the developer leaves the circumferential surface of the development sleeve  34   a . From the development range A to the developer release position  46 , the developer is kept on the surface of the development sleeve  34   a  by the first magnetic field generated by the development pole S 1  and the third magnetic field generated by the post-development pole N 1  from the development range A to the release portion  46  where the developer is separated from the development sleeve  34   a.    
     By contrast, in the five-pole configuration shown in  FIG. 6 , another pole is provided between the development pole  34   s  and the attraction pole  34   r , yet another pole is provided between the development pole  34   s  and the release pole  34   u . In this configuration, when the development roller  34  has a relatively small diameter, the development sleeve  34   a Z has a shorter interval between the release pole  34   u  and the attraction pole  34   r  disposed downstream from the releaser pole  34   u  in the rotational direction of the development roller  34   a Z, which can inhibit the developer from leaving the development roller  34   a Z. 
     In the three-pole configuration, because both the pre-development pole N 2  functioning as the attraction pole and the post-development pole N 1  functioning as the release pole are adjacent to the development pole S 1 , a longer interval can be maintained between the pre-development pole (attraction pole) N 2  and the post-development pole (release pole) N 1  disposed downstream from the pre-development pole N 2  in the rotational direction thereof. 
     Referring to  FIG. 2 , the developer  32  that has passed the development range A reaches the portion facing the circulation path  38  as described above, after which the developer leaves the development sleeve  34   a  around the release portion  46 . In the first embodiment, the circulation path  38  is disposed beneath the supply path  37 , and the development sleeve  34   a  moves upward from the position facing the pose-development pole N 1  serving as the release pole to the position facing the post-development pole N 2  serving as the attraction pole N 2 . Therefore, gravity can act on the release of the developer from the development sleeve  34   a  between the release pole (N 1 ) and the attraction pole (N 2 ). Generally, the interval between the release pole and the attraction pole decreases as the diameter of the development sleeve decreases. Accordingly, the developer that has passed the release pole might be attracted to the attraction pole, failing to leave the development sleeve. However, in the present embodiment, because the gravity can act on the release of the developer from the development sleeve  34   a , such failure can be reduced. 
     If the developer  32  does not leave the development sleeve  34   a  or the developer that has left the development sleeve  34   a  is again attracted to the development sleeve  34   a  from the circulation path  38 , the developer whose toner concentration is lower is supplied to the development range A, decreasing the image density, which is not desirable. Therefore, the developer should leave the development sleeve  34   a  around the release portion  46 . 
     Mechanism of release of the developer is as follows: The developer  32  on the development sleeve  34   a  is transported by the development sleeve  34   a  due to frictional force generated between the surface of the development sleeve  34   a  and the developer  32 . Because this frictional force is proportional to a vertical drag applied to the developer  32  on the development sleeve  34   a , this frictional force increases as the magnetic force in a normal direction acting on the developer  32  is larger in a direction to attract the developer, and accordingly the ability of the development sleeve  34   a  to transport the developer  32  increases. 
     Hereinafter, the magnetic force generated by the magnet roller  34   b , acting in the normal direction to the surface of the development sleeve  34   a  and that acting in a direction tangential to the surface of the development sleeve  34   a  are referred to as the “magnetic force in normal direction” and the “magnetic force in tangent direction”, respectively. Additionally, the magnetic force in normal direction is also referred to as “magnetic attraction in normal direction” when acting in the direction toward the center of rotation  34   p  of the development sleeve  34   a  and “magnetic repulsion in normal direction” when acting in the direction away from the center of rotation  34   p  of the development sleeve  34   a.    
     In other words, when the magnetic attraction in normal direction is larger, the developer  32  can receive a transport force from the development sleeve  34   a  and be transported as the development sleeve  34   a  rotates. By contrast, when the magnetic attraction in normal direction is smaller, the frictional force between the development sleeve  34   a  and the developer  32  is weaker, and the developer  32  is likely to slip on the development sleeve  34   a  and is less likely to be transported by the development sleeve  34   a.    
     When the magnetic attraction in normal direction is smaller than the weight of the developer  32  itself, or when the magnetic repulsion in normal direction acts on the developer  32 , the developer  32  leaves the development sleeve  34   a . When the magnetic attraction in normal direction is greater and accordingly the frictional force between the development sleeve  34   a  and the developer  32  is greater, the developer  32  can be transported at a velocity substantially identical to the rotational velocity of the development sleeve  34   a . In other words, inertial force acts on the developer  32  because the developer  32  rotates at a high velocity similarly to the development sleeve  34   a.    
     Therefore, when the magnetic attraction in normal direction is smaller in the release portion  46 , the developer  32  can leave the development sleeve  34   a  due to the inertial force in addition to the weight of the developer  32 . In other words, to separate the developer  32  from the development sleeve  34   a , the magnetic attraction in normal direction should be smaller and the inertial force or the weight of the developer  32  should be used. Alternatively, the repulsion in normal direction should be generated to separate the developer  32  from the development sleeve  34   a  magnetically. 
     Release of Developer in Respective Magnetic Poles 
       FIG. 7  is a graph illustrating a distribution of the magnetic flux density in normal direction to the surface of the development sleeve  34   a  in the three-pole configuration. 
     In  FIG. 7 , reference characters M 1 , M 2 , and M 3  respectively represent centers (e.g., pre-development magnetic flux density peak position, development magnetic flux density peak position, and post-development magnetic flux density peak position) of the pre-development pole N 2 , the development pole S 1 , and the post-development pole N 1  where the magnetic flux density in normal direction therein is maximum. Dotted lines L 1 , L 2 , and L 3  represent a pore-development center line, a development center line, and a post-development center line connecting the center of rotation  34   p  and the pre-development center M 1 , the development center M 2 , and the post-development center M 3 , respectively. 
     In  FIG. 7 , a center angle formed by the pre-development center line L 1  and the development center line L 2  is referred to as the angle θ 1 , a center angle formed by the development center line L 2  and the post-development center line L 3  is referred to as the angle θ 2 , and a center angle formed by the post-development center line L 3  and the pre-development center line L 1  is referred to as the angle θ 3 . 
     When relative positions of the post-development pole N 1  and the pre-development pole N 2  are set so that the angle θ 3  formed by the post-development center line L 3  and the pre-development center line L 1  is 180° or greater, the magnetic flux forming the release pole (N 1 ) is more likely to flow to the development pole S 1 , and thus the magnetic field generated between the release pole (N 1 ) and the attraction pole (N 2 ) can be smaller. Thus, the developer  32  can be separated from the development sleeve  34   a  reliably. 
     Therefore, in the present embodiment, because the post-development pole N 1  and the pre-development pole N 2  can be set so that the angle θ 3  formed by the post-development center line L 3  and the pre-development center line L 1  is 180° or greater, the developer can be separated from the development sleeve  34   a  reliably even when the diameter of the development sleeve is relatively small. 
       FIG. 8  illustrates the configuration of the development device  3  shown in  FIG. 2  together with a waveform of the magnetic fields around the development roller  34 . It is to be noted that, in  FIG. 8 , reference character  34   h  represents a horizontal axis passing through the center of rotation  34   p  of the development roller  34 . 
     As in the development device  3  shown in  FIGS. 2 and 8 , when the number of the magnetic poles is three and the developer  32  in the supply path  37  is poured onto the development sleeve  34   a  and transported unidirectionally, the developer  32  can be reliably supplied to the development sleeve  34   a  and transported thereby. That is, because unidirectional development devices include separate developer transport paths (circulation path  38  and supply path  37 ) to collect the developer from the development sleeve and supply the developer to the development sleeve, a longer interval can be maintained between the release portion  46  and the attraction portion  47 . 
     In particular, by setting the angle θ 3 , formed by connecting the magnetic flux peak density M 3  (shown in  FIG. 7 ) in the post-development pole N 1 , the center of rotation  34   p , and the magnetic flux peak density M 1  (shown in  FIG. 7 ) in the pre-development pole N 2 , to an angle not smaller than 180° (θ 3 ≧180°), the magnetic attraction in normal direction can be smaller in the release portion  46 , attaining reliable release of developer. 
     In the first embodiment, by setting the magnetic flux density at the center M 1 , where the magnetic flux density is maximum in the pre-development pole N 2 , in normal direction to the surface of the development sleeve  34   a  to 10 mT or greater, the force to keep the developer on the surface of the development sleeve  34   a  can be greater than the force of the developer to fall under its own weight. Thus, the developer can be prevented from falling to the circulation path  38 . 
     The magnetic fields and the release of developer when the angle θ 3  is 180° are described below with reference to  FIGS. 9A and 9B . 
       FIG. 9A  illustrates a waveform of the magnetic fields representing the degree of the magnetic flux density in the normal direction to the surface of the development sleeve  34   a  when the angle θ 3  is 180°.  FIG. 9B  illustrates movement of the developer based on the result calculated using the waveform shown  FIG. 9A . In the calculation, the diameter and the rotational velocity of the development sleeve  34   a  are respectively set to 10 mm and 200 mm/s, and the magnetic moment and the particle size of the particles are respectively set to 75 emu/g and 40 μm. In  FIG. 9A , a maximum of the magnetic flux density is 100 mT. 
     When the angle θ 3  is 180°, as shown in  FIG. 9A , the peak of the magnetic flux density in normal direction is about 5 mT in an area between the post-development pole N 1  serving as the release pole and the pre-development pole N 2  serving as the attraction pole, and the magnetic fields generated by the magnets N 1  and N 2  are not sufficiently strong for the development sleeve  34   a  to carry the developer between the post-development pole N 1  and the pre-development pole N 2 . Additionally, it is can be known form  FIG. 9B  that, after passing through the portion facing the photoconductor  1 , the developer can leave the development sleeve  34   a  at the portion facing the circulation screw  40 , that is, the developer can be separated from the developer sleeve  34   a  reliably. 
     The magnetic fields and the release of developer in a comparative example 1 in which the angle θ 3  is 150° are described below with reference to  FIGS. 10A and 10B . 
       FIG. 10A  illustrates a waveform of the magnetic fields when the angle θ 3  is 150°.  FIG. 10B  illustrates movement of the developer based on the result calculated using the waveform shown  FIG. 10A . The conditions in the calculation are similar to those used for the graph shown in  FIG. 9A  except the value of the angle θ 3 . 
     In the comparative example 1, as shown in  FIG. 10A , the peak of the magnetic flux density is about 12 mT in an area between a post-development pole N 1 Z and a pre-development pole N 2 Z. Additionally, it can be known from  FIG. 10B  that, after passing through the portion facing a photoconductor  1 Z, a certain amount of developer fails to leave the development sleeve  34   a Z 1  (hereinafter “carried-over developer”) at the portion facing a circulation screw  40 Z, and the developer is transported to a portion facing a supply screw  39 Z. Thus, reliable release of developer cannot be attained. 
     Next, comparison of the three-pole configuration and the five-pole configuration in the development sleeve  34   a  whose diameter is smaller is described below. 
     Typically, when the number of the developer-carrying magnetic pole is not less than five, the magnetic repulsion in normal direction to the surface of the development sleeve can be sufficiently strong to separate the developer from the development sleeve. When the development sleeve has a relatively large diameter, even when multiple magnets forming the magnet roller are provided inside the development sleeve, a certain amount of space can be maintained between the magnets, and thus flexibility in the arrangement of the magnets is increased. By contrast, when the number of the magnets is smaller, and the space between the magnetic fields generated by the respective magnets is excessively large, vectors of the density of the magnetic flux generated between the magnetic poles are more uneven, and thus it is difficult to transport the developer reliably. 
     In other words, when the number of the magnets is greater, the magnets are arranged at smaller intervals, and accordingly each magnet can be affected by the magnetic force generated by other magnets, failing to generate a necessary distribution of the magnetic flux density. By contrast, when the number of the magnets is smaller, the space between the magnets is excessively large, and it is difficult to transport the developer reliably. Therefore, the number of the magnets should be determined according to the diameter of the development sleeve  34   a.    
       FIG. 11  is a schematic diagram illustrating a configuration of a development device  3 Z 2  according to a comparative example 2. Differently from the development device  3  according to the first embodiment shown in  FIGS. 2 and 8 , the development device  3 Z 2  uses a five-pole development roller  34 Z formed by a development sleeve  34   a Z in which a five-pole magnet roller  34   b Z is provided. Hereinafter, although a suffix Z is added to the reference character of each component of the development device according to any one of the comparative examples, they have similar configurations to those of the first embodiment shown in  FIGS. 2 and 8  unless described otherwise, and thus descriptions thereof are omitted. 
     When a development sleeve  34   a Z has a diameter within a range from about 6 mm to 12 mm and is relatively small, because the magnets are arranged at smaller intervals, each magnet is more susceptible to the magnetic force lines generated by other magnets. In other words, to attain desired distribution of magnetic flux density, the magnets should be arranged precisely, and it is difficult to attain desired distribution of magnetic flux density in this configuration. 
     By contrast, as described above, in the three-pole configuration shown in  FIGS. 2 and 8 , the magnets can be arranged at greater intervals than the intervals in the five-pole configuration shown in  FIG. 11  even when the development sleeve  34   a  has such a relatively small diameter (e.g., 6 mm to 12 mm), which can increase flexibility in forming the distribution of the magnetic flux distribution. Simultaneously, in the three-pole configuration, the number of the magnets is sufficient to maintain proper intervals, not excessively long, between the magnets so that the developer can be transported reliably. 
     When the diameter of the development sleeve  34   a  is smaller, because the interval between the centers (M 1  and M 2 , and M 2  and M 3 ), which is the peak of magnetic flux density, of two adjacent magnetic poles is sufficiently small. Therefore, the single pre-development pole N 2  can serve as both the attraction pole and the pre-development transport pole. Similarly, the development pole S 1  and the post-development pole N 1  together perform processes of three magnetic poles, namely, the development pole, the post-development transport pole, and the release pole. Thus, the magnetic fields generated by the magnets S 1 , N 1 , and N 2  can be sufficiently strong for performing the processes of attracting the developer, transporting the developer, developing the latent image with the developer, and releasing the developer from the development sleeve, and thus these processes can be performed reliably. 
     Thus, the three-pole development roller  34  shown in  FIGS. 2 and 8  is preferred to the five-pole development roller  34 Z shown in  FIG. 11 . 
     In the present embodiment, because the three magnetic poles have functions of attracting the developer to the development sleeve, development, and releasing the developer from the development sleeve, all of the processes of attraction of the developer to the development sleeve, adjustment of the amount of the developer carried on the development sleeve, development, and release of the developer from the development sleeve can be reliably performed similarly to typical configurations in which the number of magnetic poles is five. 
     The magnetic force F (N) can be expressed using the formula below. 
     
       
         
           
             F 
             = 
             
               
                 
                   4 
                   ⁢ 
                   π 
                 
                 
                   μ 
                   0 
                 
               
               ⁢ 
               
                 
                   μ 
                   - 
                   1 
                 
                 
                   μ 
                   + 
                   2 
                 
               
               ⁢ 
               
                 
                   a 
                   3 
                 
                 8 
               
               ⁢ 
               
                 B 
                 · 
                 
                   ∇ 
                   B 
                 
               
             
           
         
       
     
     wherein the B(T) represents the magnetic flux density, μ 0  (H/m) represents the magnetic permeability at vacuum, μ represents a relative magnetic permeability of carrier particles, and a represents a particle diameter of carrier particles. 
     The graph described below illustrates the magnetic force when carrier particles have a diameter of 35 am and a relative magnetic permeability of 8 as an example. 
       FIG. 12  is a graph illustrating a comparison between the magnetic force in normal direction to the surface of the development sleeve  34   a  around the release portion  46  in the three-pole development device  3  shown in  FIG. 8  and that in the five-pole development device  3 Z 2  shown in  FIG. 11 . To make the graph shown in  FIG. 12 , the magnetic force was actually measured. 
     In  FIG. 12 , “∘” represents the magnetic force in the three-pole configuration, and “x” represents that in the five-pole configuration, a vertical axis represents the magnetic force, and a horizontal axis indicates an angle that is “0” where the horizontal axis  34   h  crosses the surface of the development sleeve  34   a  and increases in the plus direction as the angle position moves in the direction indicated by arrow D shown in  FIG. 8 . It is to be noted that, in this specification, “angle” means the center angle formed by the horizontal axis  34   h  shown in  FIG. 8  and a line connecting the center of rotation  34   p  and a given point on the surface of the development sleeve  34   a  unless otherwise specified. 
     Referring to  FIG. 12 , as the magnetic force in normal direction approaches 0 N, the magnetic force to attract the developer on the development sleeve  34   a  decreases, and accordingly the developer leaves the development sleeve  34   a  due to gravity. 
     The magnetic force in normal direction functions as the magnetic attraction when being in the minus direction. In the graph shown in  FIG. 12 , as the magnetic force in normal direction increases in the minus direction, the magnetic force to attract the developer on the development sleeve  34   a  increases, and the frictional force therebetween also increases. Accordingly, the developer is less likely to leave the development sleeve  34   a . In this case, the magnetic force serves as the magnetic attraction also in the release portion  46 , and developer fails to leave the development sleeve  34   a , which is not desirable. This phenomenon is hereinafter referred to as developer release failure. 
     By contrast, the magnetic force in normal direction functions as the magnetic repulsion when being in the plus direction. 
     In the development device  3  shown in  FIG. 8  and the comparative example shown in  FIG. 11 , the release portion  46  is disposed in an angle range from −40° to 0°. When the magnetic force was measured in the comparative example 2 using the five-pole development sleeve  34   a Z whose diameter was 10 mm, a magnetic force Fr in normal direction around the release portion  46 Z was not greater than about −4×10 −10  N (Fr≦−4×10 −10 ). Thus, developer failed to leave the development sleeve  34   a Z. 
     In this measurement, the magnetization, particle size, and density of the carrier particle was within a range from 30 emu/g to 120 emu/g, a range from 20 μm to 80 μm, and a range from 3 g/cm 3  to 8 g/cm 3 , respectively. 
     Typically, in the five-pole development device  3 Z 2 , the developer can be separated from the development sleeve  34   a Z by generating the magnetic repulsion in normal direction in the release portion  46 Z. However, when the development sleeve  34   a Z has reduced diameter within a range from about 6 mm to 12 mm, because each magnet is more susceptible to the magnetic force lines generated by other magnets, it is difficult to generate desired magnetic repulsion in normal direction. 
     In the three-pole development device  3  shown in  FIG. 8 , the release of developer in the release portion  46  can be improved by reducing the magnetic flux density in the attraction portion  47 . When the magnetic force was measured in the development device  3  shown in  FIG. 8 , developer was reliably separated from the development sleeve  34   a  when the magnetic flux density in normal direction at the center M 1  (magnetic flux density peak) of the pre-development pole N 2  was not greater than 30 mT. 
     The magnetic force lines from the post-development pole N 1  partly flow into the development pole S 1 , and the rest of the magnetic force lines pass around the release portion  46  and then return to the post-development pole N 1 . Similarly, the magnetic force lines from the pre-development pole N 2  partly flow into the development pole S 1 , and the rest of the magnetic force lines pass around the release portion  46  and then return to the pre-development pole N 2 . 
     The vector of magnetic force around the release portion  46  is determined by the balance between the magnetic force lines flowing from the post-development pole N 1  and returning thereto and that flowing from the pre-development pole N 2  and returning thereto. When the magnetic flux density in the pre-development pole N 2  is reduced, the magnetic force lines flowing from the pre-development pole N 2  to the development pole S 1  increase relatively, and the magnetic force lines that pass around the release portion  46  are decreased. As a result, the magnetic attraction in normal direction in the release portion  46  can be reduced. 
     This relation can be expressed as Br 1 &gt;Br 3 , wherein the peak of magnetic flux density in normal direction of the development pole S 1  is Br 1  (hereinafter simply “magnetic flux density peak in S 1 ”), and the peak of magnetic flux density in normal direction of the pre-development pole N 2  is referred to Br 3  (hereinafter simply “magnetic flux density peak in N 2 ”). 
       FIG. 13  is a graph illustrating the magnetic attraction in normal direction measured around the release portion  46  in the development device  3  according to the first embodiment. In the measurement, the peak of the magnetic flux density in normal direction in the pre-development pole N 2  was set to 30 mT and 60 mT. 
     The release portion  46  was set to the angle range from −40° to 0° in the direction indicated by arrow D shown in  FIG. 8 . As it is known form  FIG. 13 , when the peak of magnetic flux density in normal direction of the pre-development pole N 2  was 30 mT, which is half of 60 mT, the magnetic attraction in normal direction in the release portion  46  was substantially zero, and thus the developer was reliably separated from the development sleeve  34   a.    
     Relation Between Developer Release and Magnetic Flux Densities in Development Pole and Developer Release Pole 
     The magnetic attraction in normal direction can be reduced when the relation Br 1 &gt;Br 2  is satisfied, wherein Br 1  represents the magnetic flux density peak in S 1 , and Br 2  represents the peak of magnetic flux density in normal direction of the post-development pole N 1  (hereinafter simply “magnetic flux density peak in N 1 ”). 
       FIG. 14  is a graph comparing the magnetic attraction in normal direction around the release portion  46  measured when Br 1  and Br 2  were substantially the same and that measured when Br 1 &gt;Br 2  in a development device  3 A shown in  FIG. 26  to be described later. In this measurement, the release portion  46  was set to the angle range from 20° to 50° (hereinafter referred to “release angle”) in the direction indicated by arrow D shown in  FIG. 8 . 
     As described above, the magnetic force lines from the post-development pole N 1  partly flow into the development pole S 1 , and the rest passes around the release portion  46  and return to the post-development pole N 1 . As shown in  FIG. 14 , when Br 2  (magnetic flux density peak in N 1 ), is lower than Br 1  (magnetic flux density peak in S 1 ), the magnetic force lines flowing from the post-development pole N 1  to the development pole S 1  increase, and the magnetic force lines flowing from the post-development pole N 1  passing around the release portion  46  are decreased. As a result, the magnetic attraction in normal direction in the release portion  46  can be reduced. 
     From the descriptions above, it can be known that the magnetic attraction in normal direction in the release portion  46  can be reduced when both Br 1 &gt;Br 2  and Br 1 &gt;Br 3  are satisfied. 
     Next, the relation between Br 2  (magnetic flux density peak in N 1 ) and Br 3  (magnetic flux density peak in N 2 ) is described below. 
     As described above, the post-development pole N 1  needs to transport the developer that has been used in development. Additionally, the post-development pole N 1  needs to cause the carrier particles that have adhered to the photoconductor  1  in the development range A to be again attracted to the development sleeve  34   a . Therefore, Br 2  (magnetic flux density peak in N 1 ) should be sufficiently high to transport the developer as well as to attract the carrier particles. Also in the relation between Br 2  (magnetic flux density peak in N 1 ) and Br 3  (magnetic flux density peak in N 2 ), when either of them is to be increased, the other should be reduced to balance the magnetic force on the development sleeve  34   a  because the developer is inhibited from leaving the development sleeve  34   a  if both of them are increased, that is, magnetic force in total increases. 
     Therefore, in the present embodiment, to secure the above-described functions of the post-development pole N 1 , Br 2 , magnetic flux density peak in N 1 , should be higher than Br 3 , magnetic flux density peak in N 2  (Br 2 &gt;Br 3 ). That is, the relation Br 1 &gt;Br 2 &gt;Br 3  is satisfied. 
     Next, a comparative example in which the number of the developer-carrying magnetic pole is only one (hereinafter “single-pole configuration”) is described below. 
       FIG. 15  is a graph illustrating distribution of the magnetic flux density in normal direction at respective positions on the surface of the development sleeve in the single-pole configuration, and three different conditions are used. In  FIG. 15 , an angle of 180° on the horizontal axis represents the peak position of the magnetic flux density in normal direction. In  FIG. 15 , reference character dDEG (dDEG 1  and dDEG 2 ) represents an angle range including the peak position of the magnetic flux density in normal direction, formed by the center of rotation of the development sleeve and two positions on the circumference of the development sleeve at which the magnetic flux density in normal direction to the surface of the development sleeve is 0 mT. Conditions 1 through 3 used to obtain the graph shown in  FIG. 15  are specified in table 1 shown below. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Condi- 
                 Peak value 
                 Half hand 
                 dDEG 
                   
                 ΔZ 
                 X/ 
               
               
                 tion 
                 (-mT) 
                 width (°) 
                 (°) 
                 X 
                 (60) 
                 (360-dDEG) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 1 
                 21 
                 30.5 
                 62 
                 640.5 
                 2.1 
                 2.1 
               
               
                 2 
                 37 
                 35.5 
                 62 
                 1313.5 
                 4.1 
                 4.1 
               
               
                 3 
                 63 
                 18 
                 39 
                 1134 
                 3.3 
                 3.2 
               
               
                   
               
            
           
         
       
     
     In  FIG. 15  and table 1, ΔZ (60) represents the magnetic flux density at an angle of 60° on the circumference of the development sleeve. As shown in  FIG. 15 , under any of the conditions 1, 2, and 3, although fluctuations in the magnetic flux is significant around the peak position of magnetic flux density in normal direction, the magnetic flux is substantially constant around the angle of 60° on the circumference of the development sleeve. It is to be noted that an angle of 60° on the circumference of the development sleeve means a rough angle at which a mean value of the magnetic flux densities in plus direction in  FIG. 15  is obtained. 
     In the single-pole configuration, when it is assumed that the polarity of the peak position of the magnetic flux density in normal direction is N pole (negative direction in  FIG. 15 ), an magnetic field whose polarity is S pole (positive direction in  FIG. 15 , opposite that of the peak position) is generated in a range on the surface of the development sleeve except the range (dDEG 1  and dDEG 2 ) whose polarity is N pole, which is identical to that of the peak position. 
     At this time, the integrated density of magnetic flux in normal direction in the N pole range is identical or similar to the integrated density of magnetic flux in normal direction in the S pole range on the surface of the development sleeve. 
     A product of a peak value Br of the magnetic flux density and a half band width θh in that magnetic pole can be used as an approximation of the integrated density of magnetic flux in normal direction in the range whose polarity is identical to that of the peak position. Hereinafter, the product of the peak value Br and the half band width θh is referred to as a magnetic flux density product X. The half hand width θh means, in a magnetic field generated by a single magnetic pole, an angle in an angle range including the peak position of the magnetic flux density in normal direction, formed by the center of rotation of the development sleeve and two positions on the circumference of the development sleeve where the magnetic flux density in normal direction is half the peak value Br. 
     Because the integrated density of magnetic flux in normal direction in the range whose polarity is identical to that of the peak position is identical or similar to that in the range whose polarity is the opposite, the mean value of the magnetic flux density in normal direction in the range whose polarity is the opposite can be identical or similar to a value obtained by dividing the magnetic flux density product X by the center angle (360−dDEG) of the range whose polarity is the opposite. 
     Additionally, under any of the conditions 1 through 3 shown in table 1, the value of X/(360−dDEG) is similar to ΔZ (60), and accordingly it can be known that the integrated density of magnetic flux in normal direction in the range whose polarity is opposite that of the peak position appropriates to the product of the peak value Br and the half hand width θh. 
     As shown in table 1, the magnetic pole under condition 1 has a peak value of −23 mT and a half band width of 30.5°. Then, the product X of these values is 640.5, and the mean value of the magnetic flux density in normal direction of the opposite polarity is 2.1 mT. Similarly, under the condition 2, the product X of these values is 1313.5, and the mean value of the magnetic flux density in normal direction of the opposite polarity is 4.1 mT. Thus, the product X and the mean value of the magnetic flux density in normal direction of the opposite polarity under condition 2 are respectively 2.05 times and 1.95 times the values obtained under condition 1. That is, under the conditions 1 and 2 in which the values of dDEG are identical, the mean value of the magnetic flux density in normal direction of the opposite polarity is substantially proportional to the product X. Additionally, under the condition 3 in which the value of dDEG is different, a similar relation can be observed. 
     Although the description above concerns the single-pole configuration, in multiple magnetic-poles configurations, basically, the above-described theory works by integrating the distributions of magnetic flux density in normal direction. To be more exact, the description in the multiple-poles configuration is not so simple because it is possible that a magnetic circuit may be formed inside the development sleeve and the magnetic field does not leak to the surface. However, when the number of the magnetic poles is relatively small as in the present embodiment, an approximate graph of the distribution of magnetic flux density in normal direction can be drawn by integrating the distribution of magnetic flux density in the single-pole configuration. 
     That is, on the circumferential surface of the development roller, the integrated density of magnetic flux in normal direction whose polarity is S pole can be substantially identical to the integrated density of magnetic flux in normal direction whose polarity is N pole. Additionally, when the density peak of magnetic flux is relatively large as in the conditions 2 and 3 for  FIG. 15 , the density peak of magnetic flux of the opposite polarity, which arises around a position on the development sleeve where the direction (plus/minus) of magnetic flux in normal direction is reversed, is also relatively large.  FIG. 16  illustrates an example of distribution of magnetic flux density in normal direction around the development sleeve in which the density peak of the magnetic flux of the opposite polarity is considered. Peak values, half band widths, and products X in the respective magnetic poles, shown in  FIG. 16 , provided in the development sleeve  34   a  are specified in table 2 shown below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Condition 
                 Peak value (-mT) 
                 Half hand width (°) 
                 X 
               
               
                   
                   
               
             
            
               
                   
                 S1 
                 95 
                 48 
                 4560 
               
               
                   
                 N1 
                 90 
                 33 
                 2970 
               
               
                   
                 N2 
                 30 
                 42 
                 1260 
               
               
                   
                   
               
            
           
         
       
     
     In the present embodiment, because the density peak of magnetic flux in the post-development pole N 1  is larger, a magnetic pole S 2  whose polarity is opposite the polarity of the post-development pole N 1  is formed downstream from the post-development pole N 1  in the rotational direction of the development sleeve  34   a  as shown in  FIG. 16 . Although another S pole whose polarity is opposite the polarity of the post-development pole N 1  is formed upstream from the post-development pole N 1  in that direction, because the development pole S 1  has a peak of magnetic flux density sufficiently larger than that of that S pole, the distribution of magnetic flux density in normal direction of that S pole is integrated into the development pole S 1 . Although the density peak of magnetic flux in the development pole S 1  is also larger, because the opposite pole of the development pole S 1  is sandwiched between the post-development pole N 1  and the pre-development pole N 2  whose peak values are sufficiently larger than that of the opposite pole, the distribution of magnetic flux density in normal direction of the pole opposite the development pole S 1  is integrated into the post-development pole N 1  and the pre-development pole N 2 . 
     As shown in  FIG. 16 , if the magnetic pole S 2  is formed, the magnetic pole S 2  might serve as a developer transport pole of the developer that has passed the position facing the post-development pole, inhibiting the release of the developer from the development sleeve  34   a . Therefore, the effect of the magnetic pole S 2  should be inhibited. 
     In the present embodiment, to inhibit the effect of the magnetic pole S 2 , it is preferred that the product X of the magnetic flux density peak Br and the half hand width θh in the development pole S 1  be greater than the sum of the product (X=Br·θh) in the post-development pole N 1  and the product (X=Br·θh) in the pre-development pole N 2 . 
     Setting the product X in the development pole S 1  is described below. Hereinafter, reference characters Br 1  θh 1 , and X 1  represent the magnetic flux density peak in normal direction, the half hand width, and the magnetic flux density product, respectively, in the development pole S 1  in the present embodiment. 
     Similarly, reference characters Br 2  θh 2 , and X 2  represent the magnetic flux density peak in normal direction, the half hand width, and the magnetic flux density product, respectively, in the post-development pole N 1  whose peak of magnetic flux density is higher than that of the pre-development pole N 2  although the polarity of them are identical. Similarly, reference characters Br 3  θh 3 , and X 3  represent the magnetic flux density peak in normal direction, the half hand width, and the magnetic flux density product, respectively, in the pre-development pole N 2  whose peak of magnetic flux density is lower than that of the pre-development pole N 1 . 
     At this time, when the development roller  34  is configured so that the relations Br 1 &gt;Br 2 &gt;Br 3  and Br 1 ·θh 1 &gt;Br 2 ·θ 2 +Br 3 ·θ are satisfied, the value of X 1 −(X 2 +X 3 ) determines whether the mean value of magnetic flux density in normal direction in an area ε shown in  FIG. 16  is negative or positive. The area ε shown in  FIG. 16  is an area from the position where the magnetic flux density in normal direction is 0 mT downstream from the post-development pole N 1  and to the position where the magnetic flux density in normal direction is 0 mT upstream from the pre-development pole N 2  in the rotational direction of the development sleeve  34   a . If the value of X 1 −(X 2 +X 3 ) is negative, the polarity of the area ε shown in  FIG. 16  is S. 
     The polarity of the magnetic pole in normal direction to the surface of the development roller  34  is reversed to N in a downstream end portion in the area ε shown in  FIG. 16  in the rotational direction of the development sleeve  34   a.    
     By contrast, if the value of X 1 −(X 2 +X 3 ) is positive, the polarity of the area ε shown in  FIG. 16  is N. In other words, when X 1 &gt;X 2 +X 3  is satisfied, the polarity of the area ε shown in  FIG. 16  from the release pole (N 1 ) to the attraction pole (N 2 ) can be entirely identical (N pole) to that of the post-development pole N 1  and the pre-development pole N 2 . 
     By setting the polarity of the area from the release pole (N 1 ) to the attraction pole (N 2 ) to the polarity identical to that of the release pole (N 1 ), the developer can be better separated from the development sleeve  34   a  after passing through the development range A. 
     As described above, although the pole S 2  is formed due to a relatively high peak value in the post-development pole N 1 , the magnetic flux density caused by the pole S 2  can be reduced when X 1 &gt;X 2 +X 3  is satisfied. Even when the pole S 2  generates an S magnetic field, because its polarity can be reversed to N immediately downstream from that magnetic field, the effects by the pole S 2  can be inhibited. Therefore, fluctuations in the magnetic flux density in normal direction between the post-development pole N 1  to the pre-development pole N 2  can be reduced, which can facilitate release of the developer form the development sleeve  34   a.    
     Additionally, the position where the polarity of the magnetic field generated by the pole S 2  is reversed to N can be adjusted with the half band width of the attraction pole N 2 . 
     Developer Release Angle 
       FIG. 17  illustrates a relation between the release angle and the amount of carried-over developer when the development sleeve  34   a  rotates counterclockwise and the angle at which the horizontal axis  34   h  crosses the surface of the development sleeve  34   a  is 0° as in the first embodiment shown in  FIG. 7 . In  FIG. 17 , a vertical axis indicates changes in the amount of carried-over developer. There are two factors with which the release angle varies: changes in the magnetic force of the development roller  34 , and changes in the amount of the developer in the circulation path  38 . The data shown in  FIG. 17  were obtained in an experiment in which the release angle was varied by rotating a fixture, not shown, fixing the magnets of the magnetic roller  34   b , and simultaneously, actual release angle was observed while the amount of the developer in the circulation path  38  was varied by changing the amount of the developer in the casing  33  of the development device  3 . Experiments were performed using two different development rollers  34  whose magnetic force distributions (types A and B) were different only around the release portion  46 . In  FIG. 18A , ∘ and x respectively represent the magnetic force distribution in normal direction of the types A and B, and in  FIG. 18B , ∘, and x respectively represent the magnetic force distribution in tangent direction of the types A and B, respectively. 
     When images were output in the experiment, the developer was reliably separated from the development sleeve  34   a  in the case of the magnetic force distribution type A represented by ∘ shown in  FIGS. 18A and 18B . However, the developer release failure occurred, causing an image failure in the case of the magnetic force distribution type B represented by x shown in  FIGS. 18A and 18B . 
     As shown in  FIG. 18A , the range where the magnetic attraction in normal direction was substantially zero is around a range from −20° to 60° in the type A and around a range from 20° to 100° in the type B. 
     Although, in both types A and B, the magnetic attraction in normal direction was substantially zero around the release portion  46  that in the present embodiment is within the range from 0° to 50°, the developer was reliably separated from the development sleeve  34   a  in the type A while the developer release failure occurred in the type B. 
       FIG. 19  illustrates results of another experiment to observe the relation between the release angle and the range where the magnetic force in tangent direction is zero. In this experiment, although the magnetic force in normal direction was substantially zero around the release portion  46 , the angle range where the magnetic force in tangent direction was zero was varied. A horizontal axis in  FIG. 19  indicates, as the angle at which the magnetic force in tangent direction is zero, an angle on the surface of the development sleeve  34   a , positioned at an upstream edge of the range where the magnetic force in tangent direction is zero in the rotational direction of the development sleeve  34   a . From the graph shown in  FIG. 19 , it can be known that the developer  32  around the release portion  46  is transported close to the position where the magnetic force in tangent direction is zero. Herein, the release angle means the angle at which the developer  32  started to leave the development sleeve  34   a  in the experiment. 
     Image failure caused by the developer release failure is described below. 
     Because the circulation path  38  mainly contains the used developer whose toner concentration is lower, image density is decreased when such used developer is again transported by the development sleeve  34   a  and used in development. By contrast, the toner concentration is higher immediately after fresh toner is supplied to the circulation path  38 , and accordingly image density is higher in this case. Although it is difficult to determine whether or not the image failure was caused by developer release failure if the difference is only the image density, in the case of developer release failure, the state of the developer  32  in the circulation path  38  is reflected in resulting images, which is distinctive feature of the developer release failure. For example, when unevenness in the image density of the resulting images corresponds to the pitch of the circulation screw  40 , developer release failure can be regarded as the cause. 
     The magnetic field generated by each magnet of the magnet roller  34   b  has a specific size and a specific direction, and accordingly the magnetic force generated by the magnetic field has a specific size and a specific direction. As shown in  FIG. 19 , the developer  32  is transported close to the position where the magnetic attraction in normal direction is smaller and the vector thereof is perpendicular to the surface of the development sleeve  34   a , that is, where the magnetic force in tangent direction is zero. 
       FIG. 20  illustrates a relation between the vector of magnetic force and the distribution of the magnetic flux density in normal direction around the development roller  34 . Although the developer  32  receives magnetic force along the vector of magnetic force generated by the magnets of the magnet roller  34   b , the magnetic attraction in normal direction is smaller in the release portion  46 , and the developer does not receive the force to be transported across a range  48  where the direction of the magnetic force in tangent direction changes, that is, the magnetic force in the tangent direction is substantially zero in the range  48 . Therefore, the developer is transported only to the range  48 . 
     From the graph, it can be known that, even when the magnetic force in normal direction is extremely small, the amount of carried-over developer significantly increases when the release angle is within the range from about 50° to ° 60 because, if the developer remains on the development sleeve  34   a  until that portion of the development sleeve  34   a  reaches the upper portion of the development roller  34 , vertical repulsion from the development sleeve  34   a  increases due to the weight of the developer itself even when the magnetic force is not present at all. Accordingly, the frictional force between the development sleeve  34   a  and the developer carried thereon increases. As a result, the force of the development sleeve  34   a  to transport the developer increases, which increases the amount of carried-over developer. From the results shown in  FIGS. 17 and 19 , it can be known that the developer can be reliably released from the development sleeve  34   a  by disposing the range  48  shown in  FIG. 20 , where the magnetic force in tangent direction is zero, at an angle position not greater than 50° on the development sleeve  34   a.    
     Unidirectional Circulation and Non-Unidirectional Circulation 
     Although, in the unidirectional development device  3  shown  FIG. 2 , the developer that has passed the development range A is sent to not the supply path  37  but the circulation path  38 , a non-unidirectional development device  3 Z 3  according to a comparative example 3 is described below with reference to  FIG. 21 . 
       FIG. 21  is a schematic diagram illustrating a configuration of the non-unidirectional development device  3 Z 3 . 
     The development device  3 Z 3  is different from the development device  3  shown in  FIG. 2  in that a supply path  37 Z and a circulation path  38 Z are arranged horizontal and accordingly the shape of a partition  36 Z is different from that shown in  FIG. 2 . The developer is supplied from the supply path  39 Z to a development sleeve  34   a Z and then is returned to the supply path  39 Z after passing through a development range. 
     In the development device  3 Z 3 , similarly to the development device  3  shown in  FIG. 2 , ports, not shown, are respectively formed in an upstream end portion and a downstream end portion of a partition  36 Z to connect the supply path  37 Z and the circulation path  38 Z, and a circulation screw  40 Z transports the developer in a direction opposite the direction in which the supply screw  39 Z transports the developer. Thus, the developer is circulation between the supply path  37 Z and the circulation path  38 Z. 
     In non-unidirectional development devices such as the development device  3 Z 3  shown in  FIG. 21 , the developer that has passed through the development range A is collected in the identical developer transport path from which the developer is supplied to the development sleeve  34   a Z 1 . That is, the release portion  46 Z where the developer leaves the development sleeve  34   a Z 1  and the attraction portion  47 Z where the developer is attracted to the development sleeve  34   a Z 1  are disposed in the identical developer transport path. Therefore, in particular, when the diameter of the development sleeve  34   a Z 1  is smaller and the release portion  46 Z and the attraction portion  47 Z are close to each other, it is possible that developer release failure and/or improper attraction of the developer occur more frequently. 
     Additionally, as in the comparative example 3 shown in  FIG. 21 , in the configuration in which the magnetic force generated by the magnet roller  34   b Z 1  attracts the developer to the development sleeve  34   a Z 1 , the developer can be reliably attracted to the development sleeve  34   a Z 1  when the magnetic force in the attraction portion  47 Z is sufficiently strong. However, the magnetic force is also strong in the release portion  46 Z that is close to the attraction portion  47 Z, which increases the frequency of developer release failure. By contrast, when the magnetic force in the attraction portion  47  is reduced, although developer release failure can be inhibited because the magnetic force in the release portion  46 Z is also reduced, it is possible that the developer cannot be reliably attracted to the development sleeve  34   a.    
     Therefore, it is necessary to reduce the effects of either of the release portion  46  and the attraction portion  47  given to the other by disposing them away from each other to achieve the functions of both of them. 
     Relative Positions of Development Sleeve and Circulation Screw 
     Next, relative positions of the development sleeve  34   a  and the circulation screw  40  are described below. In the unidirectional developer device  3  shown in  FIG. 2 , as described above, the amount of the developer in the circulation path  38  increases toward downstream in the developer transport direction, and the developer is likely to reach a highest edge of the circulation screw  40  in the downstream portion. Additionally, as shown in  FIG. 17 , if the developer is carried to the upper portion of the development sleeve  34   a , developer release failure is likely to occur, which is not desirable. Therefore, as the relative positions of the development roller  34  and the circulation screw  40 , the highest edge of the circulation screw  40  is preferably lower than an angle of 50° and more preferably lower than an angle of 0° on the surface of the development sleeve  34   a.    
     Relations between the toner concentration and positions in the axial direction in the supply path  37  and the circulation path  38  and on the development sleeve  34   a  are described below according to  FIGS. 22A and 22B . 
       FIG. 22A  and  FIG. 22A  are respectively graphs illustrating relations between the toner concentration in weight percent and the positions in the development device  3  shown in  FIG. 2  and that in a development device according to a comparative example 4. The graphs shown in  FIGS. 22A and 22B  were obtained by forming solid images on A3 sheets and measuring the toner concentration in the development device  3 . The amount of toner adhering to the A3 sheets was 0.45 g/cm 2 . 
     Similarly to the development device  3  according to the first embodiment, the development device according to the comparative example 4 includes a supply path provided with a supply screw and a circulation path provided with a circulation screw to transport the developer in a direction opposite the developer transport direction of the supply screw. However, the comparative example 4 is different from the first embodiment in that the developer that has passed through the development range is collected in the supply path, and only the developer that reaches a downstream end portion in the supply path in the developer transport direction is sent to an upstream end portion of the circulation path. 
     It is to be noted that the graph of the toner concentration on the developer sleeve shown in  FIGS. 22A and 22B  shows toner concentration of the developer that has just passed a position facing the developer regulator  35 . In  FIGS. 22A and 22B , as the value of vertical axes increases, the position on the supply screw shifts downstream while the position on the circulation screw shifts upstream in the developer transport direction. 
     In the first embodiment, as shown in  FIG. 22A , the toner concentration on the development sleeve  34   a  is substantially constant regardless of the position in the axial direction. By contrast, as shown in  FIG. 22B , the toner concentration on the development sleeve decreases as the position shifts downstream in the developer transport direction of the supply screw in the comparative example 4. 
     It is to be noted that, because the capacity of the developer container (e.g., supply path and circulation path) is smaller in a compact development device, the proportion of the toner in the developer decreases more significantly in it even when the identical amount of toner is consumed in the compact development devices and a typical development device. Therefore, in the comparative example in which the used developer is collected in the supply path, when the development device is relatively compact, the toner concentration on the development sleeve decreases more significantly as the position shifts downstream in the developer transport direction of the supply screw, which is not desirable. 
     By contrast, in the first embodiment in which the used developer is collected in the circulation path  38  separately provided from the supply path  37 , even when it is relatively compact and accordingly the capacity of the development container is smaller, a substantially constant toner concentration can be maintained on the development sleeve  34   a . Thus, image density can be kept substantially constant even when the development device is relatively compact. By increasing the interval between the pre-development pole (attraction pole) N 2  and the post-development pole (release pole) N 1 , the developer that has passed through the development range can be collected in the circulation path  38  separately provided from the supply path  37 . 
     Stress 
     Additionally, in the configuration in which the rotary shaft of the supply screw  39  is disposed above the center of rotation  34   p  and the developer is supplied onto the development sleeve  34   a  from above, the developer that has strode the barrier  43  can fall on the development sleeve  34   a  due to gravity. Thus, the developer can be reliably supplied to the development sleeve  34   a  even when the magnetic flux density in the attraction portion  47  is reduced. 
     This configuration is efficient when the relation Br 1 &gt;Br 2 &gt;Br 3  is satisfied. That is, when this relation is satisfied in the three-pole magnet roller  34   b , the peak Br 2  in the post-development pole N 1 , functioning as the release pole, can be used to catch carrier particles and to facilitate the release of the developer from the development sleeve  34   a  simultaneously. Although the peak Br 3 , that is, ability to attract the developer, of the pre-development pole N 2  (attraction pole) is reduced accordingly, this ability can be supplemented by supplying the developer to the development sleeve  34   a  from above. 
     Additionally, reducing the magnetic flux density in the attraction portion  47 , that is, reducing the magnetic flux density peak of the pre-development pole N 2 , can significantly reduce the stress given to the developer upstream from the developer regulator  35  in the rotation direction of the development sleeve  34   a , thus expanding the life of the developer. Reducing the stress to the developer upstream from the developer regulator  35  also can reduce load to the development sleeve  34   a , and thus deformation of the development sleeve  34   a  can be prevented or reduced even when its diameter is smaller and accordingly its strength is reduced. As described above, in the first embodiment, the relative positions of the barrier  43  forming the supply path  37  and the development sleeve  34   a  are set so that gravity can be used to supply the developer to the development sleeve  34   a . By arranging the surface of the development sleeve  34   a  below the barrier  43  as shown in  FIG. 2 , the developer that has sent over the barrier  43  as the supply screw  39  rotates can be supplied onto the development sleeve  34   a  due to gravity. 
     It is to be noted that, by using gravity to supply the developer, even in the present embodiment in which an identical magnetic pole serves as both the attraction pole and the developer regulation pole, that is, the number of the magnetic poles is reduced from that in the configuration that includes the attraction pole and the developer regulation pole separately, the developer can be supplied to the development range reliably. 
     In the first embodiment, among the magnetic poles (S 1 , N 1 , and N 2 ) for carrying the developer onto the development sleeve  34   a , the magnetic flux density peak of the magnetic field generated by the pre-development pole N 2  in normal direction to the surface of the development sleeve  34   a  is 40 mT or less. Thus, this magnetic flux density peak of the magnetic field generated by the pre-development pole N 2  is reduced from a conventional configuration in which the magnetic flux density peak of the magnetic field generated by the attraction pole is about 50 mT to 70 mT. 
     In particular, the load to the development sleeve  34   a  as well as the stress to the developer thereon can be reduced by setting the magnetic flux density in normal direction at the center M 1  (magnetic flux density peak) of the pre-development pole N 2  to a value not greater than 30 mT. The relation between the attraction portion  47  and the size of the magnetic flux density in the pre-development pole N 2  should be set in view of the following condition: The barrier  43  to be strode by the developer should be configured so that the developer can be carried on the development sleeve  34   a  due to the magnetic force in the pre-development pole N 2  and the frictional force between the developer and the development sleeve  34   a . Additionally, the position where the developer where the developer is caused to fall is the above-described position where the developer can be carried by the development sleeve  34   a  and the position between the post-development pole N 1  and the pre-development pole N. 
     When gravity is used to supply the developer onto the surface of the development sleeve  34   a , the developer can be transported reliably even when the maximum magnetic flux density of the magnetic field generated by the pre-development pole N 2  serving as the attraction pole in the normal direction to the development sleeve  34   a  is reduced to about one fourth of that in typical development devices. In this case, the load to the development sleeve  34   a  can be reduced to 20 percent to 30 percent of that in typical development devices. 
     Thus, in the present embodiment, because the load to the development sleeve  34   a  in the developer regulation portion is reduced, unevenness in the development gap, which can occur when the diameter of the development sleeve  34   a  is smaller, can be reduced, and thus image development can be performed reliably. 
     The strength of the development sleeve  34   a  decreases as the diameter thereof decreases, which is described in further detail below. 
       FIG. 23  is a graph illustrating the deformation amount of the development sleeve  34   a  when an aluminum sleeve (represented by “Al” in  FIG. 23 ) having a diameter of 10 mm and a stainless steel sleeve (represented by “SUS” in  FIG. 23 ) having a diameter of 10 mm is used, and the deformation amount of an aluminum sleeve according to a comparative example, having a diameter of 18 mm and a thickness of 0.8 mm. In  FIG. 23 , a horizontal axis indicates the thickness of the sleeves whose diameter is 10 mm, and a vertical axis indicates their deformation rates when the deformation amount of the sleeve according to the comparative example 5 is 1. 
     It is to be noted that the deformation amount, that is, the amount by which the sleeve is deformed, herein means a maximum deformation amount δmax (e.g., deformation amount of a center portion) when uniformly-distributed load is given to either end of the support beam. The maximum deformation amount δmax can be calculated using formula 1 shown below. 
     
       
         
           
             
               
                 
                   
                     δ 
                     max 
                   
                   = 
                   
                     
                       5 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         wl 
                         4 
                       
                     
                     
                       384 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       EI 
                     
                   
                 
               
               
                 1 
               
             
           
         
       
     
     wherein w represents a load per unit length, l represents the length of the development sleeve, and E represents Young&#39;s modulus, and I represents second moment of area of the development sleeve. 
     In formula 1, the Young&#39;s moduli of aluminum, ordinary steel, and stainless steel (SUS) are set to 69090 MPa, 205800 MPa, and 199920 MPa, respectively. The second moment of area of the development sleeve I is calculated using formula 2 shown below. 
     
       
         
           
             
               
                 
                   I 
                   = 
                   
                     
                       π 
                       64 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           d 
                           4 
                         
                         - 
                         
                           di 
                           4 
                         
                       
                       ) 
                     
                   
                 
               
               
                 2 
               
             
           
         
       
     
     wherein d represents the outer diameter of the development sleeve, and di represents the inner diameter of the development sleeve. 
     In the case of the aluminum sleeve having a diameter of 10 mm and a thickness of 0.7 mm, the deformation rate is 7 as shown in  FIG. 23 , that is, seven times the deformation amount of the sleeve according to the comparative example 5. Even when the SUS sleeve whose diameter and thickness are identical is used, the deformation amount is more than twice the deformation amount of the sleeve according to the comparative example 5. 
     It is to be noted that the development sleeve  34   a  is deformed by the load caused by the developer accumulated upstream from the position facing the developer regulator  35 . More specifically, the action of the accumulated developer to expand the space between the development sleeve  34   a  and the casing  33  as well as the weight of the developer push the development sleeve  34   a  to the direction opposite the developer regulator  35 . When the developer sleeve  34   a  is deformed by this load, a regulation gap, which is the space between the developer regulator  35  and the surface of the development sleeve  34   a , is larger in the center portion in the axial direction than in the end portions. Accordingly, the amount of developer passing through the regulation gap is larger in the center portion in the axial direction than in the end portions, and thus the amount of developer transported to the development range A is not uniform in the axial direction. Also in the development range A, a development gap, which is the gap between the photoconductor  1  and the development sleeve  34   a , is expanded by the developer in the center portion, and thus the development gap is uneven in the axial direction, resulting in unevenness in the image density. 
     As described above with reference to  FIG. 23 , when the diameter of the development sleeve is smaller, the deformation amount increases, causing unevenness in image density. However, in the first embodiment, the deformation of the development sleeve  34   a  can be reduced even when its diameter is relatively small because the load in the portion where the developer is regulated can be reduced by setting the magnetic flux density in the pre-development pole N 2  serving as the attraction pole to a relatively small value. Therefore, unevenness in image density can be reduced. 
       FIG. 24  is a graph illustrating torque of the development sleeve in the first embodiment and a comparative example 6. The magnetic flux density in normal direction at the center M 1  (e.g., magnetic flux density peak) of the pre-development pole N 2  in the first embodiment is 30 mT, and that in the comparative example 6 is 57 mT similarly to that in the attraction pole in typical development devices. As shown in  FIG. 24 , because the torque of, that is, the load to, the development sleeve  34   a  in the first embodiment is about 20% of that in the comparative example 6, the deformation of the development sleeve is not increased even when the aluminum sleeve having a relatively small diameter is used. 
       FIG. 25  is a graph illustrating the load calculated based on the magnetic attraction when the magnetic flux density in normal direction at the center M 1  of the pre-development pole N 2  is varied by changing the magnet forming the pre-development pole N 2  in the development device  3  shown in  FIGS. 2 and 8 . 
     In  FIG. 25 , a vertical axis indicates a total load and a horizontal axis indicates the thickness of the development sleeve  34   a . In  FIG. 25 , the load is reduced significantly when the magnetic flux density in normal direction at the center M 1  of the pre-development pole N 2  is not greater than 30 mT. Thus, the load can be reduced in the first embodiment even when the diameter of the development sleeve  34   a  is smaller and the strength thereof is smaller accordingly. 
     Attraction Pole 
     Second Embodiment 
       FIG. 26  illustrates a schematic configuration of a development device  3 A according to a second embodiment of the present invention. Except the differences described below, the development device  3 A according to the second embodiment has a similar configuration to that of the development device  3  shown in  FIG. 2  according to the first embodiment, and thus the description of the similar configuration is omitted. 
     Although the magnet S 1  is disposed so that the heights of the development pole S 1  and the center of rotation  34   p  of the development sleeve  34   a  are substantially similar, a development roller  341  shown in  FIG. 26  is different from the development roller  34  in that, in a magnet roller  34   b   1 , the development pole  51  is lower than the center of rotation  34   p  of the development sleeve  34   a . Additionally, although the magnet N 2  forming the pre-development pole N 2  is disposed so that the pre-development pole N 2  faces a highest point  34   t , which is a point on the surface of the development sleeve  34   a  at the tope of its rotation, in the first embodiment, in the second embodiment shown in  FIG. 26 , the magnet N 2  is disposed so that the pre-development pole N 2  faces a position downstream from the highest point  34   t  on the surface of the development sleeve  34   a  in the rotational direction of the development sleeve  34   a . In addition, the shape of a barrier  43 A is different from that of the barrier  43  shown in  FIG. 2 . 
       FIG. 27  illustrates the configuration of the development device  3 A shown in  FIG. 26  together with a waveform of the magnetic fields around the development roller  341 .  FIG. 28  is a graph illustrating a distribution of the magnetic flux density in normal direction in the development device  3 A shown in  FIG. 24 . 
     Third Embodiment 
       FIG. 29  illustrates a schematic configuration of the development device  3 B according to a third embodiment of the present invention. 
     Except that a separation plate  49  is provided, the development device  3 B according to the third embodiment has a similar configuration to that of the development device  3 A according to the second embodiment, and thus the descriptions thereof are omitted. 
     In the development device  3 B shown in  FIG. 29  according to the third embodiment, by providing the separation plate  49  adjacent to the release portion  46  to separate the developer  32  from the development sleeve  34   a , the developer  32  can be reliably separated from the development sleeve  34   a  even when the amount of the developer  32  is increased on the downstream side in the transport direction of the circulation screw  40 . An edge portion of the separation plate  49  is preferably close to an angle position within a range from −20° to +5° on the surface of the development sleeve  34   a  when the angle position where the magnetic force in tangent direction is zero. In other words, the edge portion of the separation plate  49  is preferably disposed close to a range on the surface of the development sleeve  34   a  downstream form the center M 1  (magnetic flux peak position) in the post-development pole N 1  and upstream from the center M 1  (magnetic flux peak position) in the pre-development pole N 2  in the rotational direction of the development sleeve  34   a.    
     It is to be noted that the configuration and position of the separation plate  49  is not limited to those shown in  FIG. 30 . Alternatively, a separation plate  49 A shown in  FIG. 30  may be used. A development device  3 B 1  shown in  FIG. 30  has a configuration similar to that of the development device  3 B shown in  FIG. 29  except the separation plate  49 A, and thus the description thereof is omitted. 
     Fourth Embodiment 
       FIG. 31  illustrates a schematic configuration of the development device  3 C according to a fourth embodiment of the present invention.  FIG. 32  illustrates the configuration of the development device  3 C shown in  FIG. 31  together with a waveform of the magnetic fields around the development roller  341 . 
     Except that a supply position adjuster  81  is provided, the development device  3 C according to the fourth embodiment has a similar configuration to that of the development device  3 A according to the second embodiment, and thus the descriptions thereof are omitted. 
     It is to be noted the distribution of the magnetic flux density in normal direction in the development device  3 C according to the fourth embodiment is similar to that shown in  FIG. 28 . Also in the fourth embodiment, the pole S 1  facing the photoconductor  1  serves as the development pole, and the pole N 1  and the pole N 2  respectively disposed downstream and upstream from the development pole S 1  in the rotational direction of the development sleeve  34   a  serve as the post-development pole and s the pre-development pole. Additionally, as shown in  FIGS. 31 and 32 , the portion where the developer  32  overflowing from the supply path  37  contacts the surface of the development sleeve  34   a  is referred to as the attraction portion  47 . 
     Generally, the developer inside the development device  3 C receives a large pressure upstream from the developer regulation portion and then deteriorates. More specifically, the amount of the developer transported to the development range A through the regulation gap is significantly small compared with the amount of the developer supplied to the development sleeve  34   a . Therefore, the developer accumulates upstream from the regulation gap in the rotational direction of the development sleeve  34   a  and accordingly receives a large pressure. 
     Additionally, the developer  32  supplied by the supply screw  39  is carried on the surface of the development sleeve  34   a  due to the magnetic attraction of the pre-development pole N 2  generated by the magnet disposed close to the position facing the supply screw  39 . At this time, if the supply screw  39  is lower than the development sleeve  34   a , the developer  32  in the supply path  37  should be carried upward to the developer sleeve  34   a  against the weight of the developer itself. Therefore, the pre-development pole N 2  serving as the attraction pole needs a relatively high magnetic flux density to supply the developer to the development sleeve  34   a  reliably. 
     The longer the distance between the development sleeve  34   a  and the supply screw  39  is, or the lower the height of the supply screw  39  relative to the development sleeve  34   a  is, the higher the magnetic flux density in the pre-development pole (attraction pole) N 2  should be to supply the developer to the development sleeve  34   a  reliably. However, although the developer can be reliably supplied to the development sleeve  34   a  by increasing the magnetic flux density in the attraction pole N 2 , the developer is more likely to deteriorate because the frictional force between the development sleeve  34   a  and the developer carried thereon increases, which is not desirable. 
     Therefore, if the developer can be reliably supplied to the development sleeve  34   a  even when the magnetic force of the attraction pole N 2  is weaker, deterioration of the developer can be reduced while reliably supplying the developer to the development sleeve  34   a.    
     As in the first, second, and third embodiments, when the supply screw  39  is disposed above the development sleeve  34   a , even when the magnetic force of the attraction pole N 2  is weaker, the developer overflowing from the supply path  37  by rotation of the supply screw  39  can fall onto the development sleeve  34   a  due to gravity. Thus, a constant amount of developer can be reliably supplied to the development sleeve  34   a.    
     Additionally, in the fourth embodiment, the supply position adjuster  81  is disposed so that the attraction portion  47  is adjusted to a position downstream from the highest position  34   t  on the surface of the development sleeve  34   a . When the magnetic flux density around the attraction portion  47  is smaller, although the developer can be supplied to the development sleeve  34   a  due to gravity, the force to keep the developer on the surface of the development sleeve  34   a  against gravity is weaker, and it is possible that a certain amount of the developer might pass between the development sleeve  34   a  and the partition  36  forming the bottom surface of the supply path  37 , falling into the circulation path  38 . However, the supply position adjuster  81  disposed as described above can prevent or reduce such inconvenience. 
     In the fourth embodiment, the supply position adjuster  81  guides the developer that has strode the barrier  43 A to the position downstream from the highest point  34   t  on the surface of the development sleeve  34   a  in the rotational direction thereof. In other words, as shown in  FIG. 28 , when the angle position where the horizontal axis  34   h  crosses the surface of the development sleeve  34   a  is 0°, by disposing the attraction portion  47  at a position not lower than 90°, the developer supplied from the supply path  37  does not fall into the circulation path  38  but can be sent to the developer regulator  35 . 
     Thus, when the casing  33  serving as the developer containing part and the development sleeve  34   a  are disposed so that gravity can be used to supply the developer from the supply path  37  onto the development sleeve  34   a  and the attraction portion  47  where the developer supplied from the supply path  37  contacts the surface of the development sleeve  34   a  is positioned downstream from the highest position  34   t  on the surface of the development sleeve  34   a , the developer supplied from the supply path  37  does not fall into the circulation path  38  but can be sent to the developer regulation portion. 
     It is to be noted that it is not preferable that a relatively large gap is present between the developer regulator  35  and the attraction portion  47 , which herein means the position where a downstream edge portion of the supply position adjuster  81  faces the development sleeve  34   a , because a larger amount of developer is present in a space between the developer regulator  35  and the barrier  43 A (hereinafter “buffer area”). In other words, if the amount of developer present in the buffer area is excessive, the amount of developer accumulating upstream from the regulation gap in the rotational direction of the development sleeve  34   a  increases, which can accelerate deterioration of the developer. 
     Additionally, because the developer accumulated in the buffer area presses the development sleeve  34   a  with its own weight, the development sleeve  34   a  is deformed particularly in the center portion in the axial direction. 
     From an experiment in which the position of the attraction portion  47  was varied by changing the shape of the supply position adjuster  81 , it is known that a center angle formed by the attraction portion  47  and position on the surface of the development sleeve  34   a  facing the developer regulator  35  is preferably not greater than 30°. 
     Fifth Embodiment 
       FIG. 33  illustrates a schematic configuration of a development device  3 D according to a fifth embodiment of the present invention. In the fifth embodiment, although the attraction portion  47  is upstream from the highest position  34   t  similarly to the first embodiment shown in  FIG. 2 , the pre-development pole N 2  is disposed close to the attraction portion  47  differently from the first embodiment. Except that, the development device  3 D has a similar configuration to that of the development device  3  shown in  FIG. 2 , and thus the descriptions thereof are omitted. 
     In this embodiment, gravity is used to supply the developer from the supply path  37  onto the development sleeve  34   a , and the attraction portion  47  is positioned upstream from the highest point  34   t  on the development sleeve  34   a  in the rotational direction of the development sleeve  34   a , by disposing the magnet forming the pre-development pole N 2  so that the peak of the magnetic flux density in normal direction of the pre-development pole N 2  is close to the attraction portion  47 . In this configuration, also the developer can be reliably supplied to the development sleeve  34   a . In an experiment, the peak of the magnetic flux density in normal direction of the pre-development pole N 2  was varied and the amount of developer fallen into the circulation path  38  was measured. In the experiment, in an angle range on the development sleeve  34   a  not greater than 80° to the horizontal axis  34   h , when the peak of the magnetic flux density in normal direction is not greater than 10 mT, the force of the developer to fall under gravity is greater than the force generated by the attraction pole to keep the developer in the development sleeve  34   a . As a result, the amount of the developer falling into the circulation path  38  increased. Therefore, it is preferred that the magnetic flux density in normal direction in the attraction portion  47  be greater than 10 mT. 
     It is to be noted that, the position of the peak of the magnetic flux density of the pre-development pole N 2  is not necessarily identical to that of the attraction portion  47 . 
     Sixth Embodiment 
       FIG. 34  illustrates a schematic configuration of a development device  3 E according to a sixth embodiment of the present invention. 
     The development device  3 E according to the sixth embodiment is different from the development device  3 D shown in  FIG. 33  in that the positions of the supply screw  39  and the circulation screw  40  relative to a development roller  342  is different and that positions of the magnets S 1 , N 1 , and N 2  are different from those in  FIG. 33 . Thus, the development roller and the magnet roller are given reference characters  343  and  34   b   3 , respectively. The development roller  343  is disposed not on the side of but beneath the photoconductor  1  in  FIG. 34 . Other than those features, the development device  3 E has a similar configuration to that of the development device  3  shown in  FIG. 2 , and thus the descriptions thereof are omitted. 
     As shown in  FIG. 34 , when the attraction portion  47  is disposed close to an angle of 0° on the surface of the development sleeve  34   a , it is preferable that the peak of the magnetic flux density in normal direction of the pre-development pole N 2  (attraction pole) and the attraction portion  47  be disposed at a substantially identical position within a range from −5° to 15°. With this configuration, the developer can be reliably supplied to the development sleeve  34   a , and simultaneously, the developer that is not carried on the development sleeve  34   a  can fall into the supply path  37  not the circulation path  38 . 
     It is to be noted that, regarding shortage of the developer, the development devices  3  through  3 D according to the first through fifth embodiments are more advantageous than the development device  3 E according to the sixth embodiment because the developer cannot overstride the barrier  43  and cannot be supplied to the development sleeve  34   a  unless a certain amount of developer is accumulated in the supply path  37  in the development device  3 E shown in  FIG. 34 . However, regarding release of the developer, the development device  3 E according to the sixth embodiment is more advantageous than the first through fifth embodiments because the position of the release portion  46  in the sixth embodiment allows the developer to fall from the development sleeve  34   a  due to its own weight. 
     Seventh Embodiment 
       FIG. 35  illustrates a schematic configuration of a development device  3 F according to a seventh embodiment of the present invention. The development device  3 F shown in  FIG. 35  is different from the development device  3  shown in  FIG. 2  according to the first embodiment in that a buried member  82  is provided upstream from the attraction portion  47  in the rotational direction of the development sleeve  34   a . Except that, the development device  3 F according to the seventh embodiment has a similar configuration to that of the development device  3  shown in  FIG. 2 , and thus the descriptions thereof are omitted. 
     Even when the attraction portion  47  is disposed upstream from the highest point  34   t  on the development sleeve  34   a  in the rotational direction of the development sleeve  34   a , as shown in  FIG. 35 , by providing the buried member  82  on the partition  36  to reduce a gap between the partition  36  forming the supply path  37  and the surface of the development sleeve  34   a , the developer can be prevented from falling into the circulation path  38 . 
     When the attraction portion  47  is disposed within an angle range from 70° to 80° on the surface of the development sleeve  34   a , by disposing the buried member  82  across a gap of about 0.5 mm to 1 mm from the surface of the development sleeve  34   a , the developer can be prevented from falling into the circulation path  38 . The buried member  82  is preferably formed of a soft material such as urethane because the development sleeve  34   a  might wear from the contact with the buried member  82 . 
     Additionally, when the attraction portion  47  is disposed within an angle range from 80° to 90° on the surface of the development sleeve  34   a , the developer can be prevented from falling into the circulation path  38  by setting the size of the gap between the buried member  82  and the development sleeve  34   a  to about 1 mm to 3 mm. This configuration is preferable because reliable developer supply can be achieved while preventing the wear of the development sleeve  34   a.    
     It is to be noted that, although the buried member  82  is provided on the partition  36  to prevent the developer from falling into the circulation path  38  in the seventh embodiment, alternatively, this objective can be achieved by reducing the size of the gap between the partition  36  and the surface of the development sleeve  34   a  when it can be set precisely. 
     When the attraction portion  47  is disposed within an angle range from 70° to 80° on the surface of the development sleeve  34   a , the developer can be prevented from falling into the circulation path  38  by setting the size of the gap between the partition  36  and the development sleeve  34   a  to about 0.5 mm to 1 mm. Additionally, when the attraction portion  47  is disposed within the angle range from 80° to 90° on the surface of the development sleeve  34   a , by setting the size of the gap between the partition  36  and the development sleeve  34   a  to about 1 mm to 3 mm, the developer can be prevented from falling into the circulation path  38  while preventing the wear of the development sleeve  34   a.    
     Variation 1 
       FIG. 36  illustrates a schematic configuration of a development device  3 C 1  according to a first variation that is a variation of the fourth embodiment (development device  3 C) shown in  FIG. 31 . Except that the development device  3 C 1  is a reverse development type, that is, the photoconductor  1  and the development roller  34   a  rotate in opposite directions in the development range A, the development device  3 C 1  according to the first variation has a similar configuration to that of the development device  3 C according to the fourth embodiment, and thus the descriptions thereof are omitted. 
     Generally, image failure in which leading edges of images are absent tends to occur in the reverse development type due to the following reason. 
     Herein, a development nip and the development range A respectively mean an area where the developer on the development sleeve  34   a  contacts the photoconductor  1  and a portion within the development nip where development is performed due to the magnetic field. While the photoconductor  1  and the development sleeve  34   a  rotate in the opposite directions, the toner adhering to the photoconductor  1  in the development range A exits the development nip. The carrier particles in the developer carried on the development sleeve  34   a  form a magnetic brush, and, upstream from the development range A in the rotational direction of the development sleeve  34   a , toner particles adhering to an edge portion of the magnetic brush move toward the development sleeve  34   a  due to the electrical field applied to a non-image area. Because a positive electrical field (hereinafter “counter charge”) remains on the edge portion of the magnetic brush, the toner adhering to the photoconductor  1  can be electrically removed therefrom by the counter charge, and thus leading edges of resultant images tends to be absent. Additionally, the development sleeve  34   a  rotating in the direction opposite the rotational direction of the photoconductor  1  can remove the toner adhering to the photoconductor  1  mechanically. 
     Even in the reverse development type development device  3 C 1 , similarly to the above-described various embodiments, the image failure in which leading edges of images are absent can be inhibited by reducing the diameter of the development sleeve  34   a  because, when the diameter is smaller, curvature of the development sleeve  34   a  increases, attaining the following advantage. 
     In the development sleeve  34   a  whose diameter is smaller, the width of the development nip is significantly small. For example, the width of the development nip may be within a range from about 1.5 mm to 2.5 mm when the diameters of the development sleeve  34   a  and the photoconductor  1  are 10 mm and 30 mm, respectively, a smallest gap between the development sleeve  34   a  and the photoconductor  1  is 0.35 mm, and the amount of developer carried on the development sleeve  34   a  that has passed through the regulation gap is 50 mg/cm 2 . When the diameter of the development sleeve  34   a  is 18 mm and other conditions are similar, the width of the development nip may be within a range from about 4 mm to 5 mm. Thus, when the diameter of the development sleeve  34   a  is smaller, the width of the development nip is significantly small. Accordingly, the magnetic brush can be separated from the photoconductor  1  immediately after development, which can reduce the amount of developer mechanically removed from the photoconductor  1 . Additionally, the size of the electrical field outside the development range A can be smaller when the curvature of the development sleeve  34   a  is larger, which can reduce the amount of toner electrically removed from the photoconductor  1 . 
     Additionally, in the reverse development type, because the sliding between the photoconductor  1  and the development sleeve  34   a  can be enhanced, the photoconductor receives less effected by after images, which can obviate the need of a cleaning unit to clean the surface of the photoconductor  1 . 
     Thus, by using the development sleeve  34   a  whose diameter is smaller in the reverse development type development device  3 C 1 , the above-described image failure can be inhibited while the cleaning unit for the photoconductor  1  is omitted, which can reduce the size and the cost of the apparatus. 
     Herein, when the developer is supplied to the development sleeve  34   a  from above as in the first through seventh embodiments and the first variation, the amount of developer that passes through the regulation gap can be substantially constant regardless of whether the developer regulator  35  is formed of a magnetic material or non-magnetic material. 
       FIGS. 37A and 37B  are graphs showing the relations between the amount (ρ) of developer that passes through the regulation gap and the width (Gd) of the regulation gap between the edge of the developer regulator  35  and the surface of the development sleeve  34   a  in the development device  3 C (shown in  FIG. 31 ) according to the fourth embodiment.  FIGS. 37A and 37B  respectively illustrate results obtained when the peak of the magnetic flux density in normal direction in the pre-development pole N 2  serving as the developer regulation pole was set to 15 mT and 30 mT. 
     As shown in  FIGS. 37A and 37B , regardless of whether the material of the developer regulator  35  is magnetic or non-magnetic, the graph showing the relation between the width Gd of the regulation gap and the amount ρ of the developer has a similar inclination, that is, changes in the amount ρ according to changes in the width Gd of the regulation gap are similar. 
     Next, transportation of the developer in the development devices  3  through  3 F (hereinafter correctively “development device  3 ”) in the first through seventh embodiments is described in further detail below.  FIG. 38  illustrates a flow and a distribution of the developer in the developer transport paths (supply path  37  and circulation path  38 ) in the development device  3  in the first through seventh embodiments. A bring-up portion  41   a  means a portion where the developer is brought up against gravity from the circulation path  38  through the bring-up port  41  to the supply path  37 . A falling portion  42   a  means a portion where the developer falls from the supply path  37  through the falling port  42  to the circulation path  38 . Therefore, the developer tends to accumulate around the bring-up portion  41   a . Herein, leakage and carrying over of the developer, which can occur on the downstream side in the circulation path  38  in the developer transport direction, are described below. As shown in  FIG. 38 , the amount per unit time of the developer brought up through the bring-up port  41  is larger than that of the developer falling through the falling portion  42   a  to the circulation path  38 . Therefore, the developer tends to accumulate around the bring-up portion  41   a . As a result, the developer tends to accumulate not only around the bring-up portion  41   a  but also upstream from the bring-up portion  41   a  in the circulation path  38  in the developer transport direction of the circulation screw  40 . 
     In the state shown in  FIG. 38 , the developer accumulates upstream from the bring-up portion  41   a  in the circulation path  38  in the developer transport direction of the circulation screw  40 . In the state shown in  FIG. 38 , the developer cannot be collected from the development sleeve  34   a  to the circulation path  38  in an area α shown in  FIG. 38 . In this case, because the developer carried on the development sleeve  34   a  that has passed through the development range A cannot enter the circulation path  38 , the developer has to leak through a gap in the casing  33  outside the development device  3 , which is hereinafter referred to as leakage of developer. 
     Additionally, as shown in  FIG. 38 , when the developer accumulates around the downstream end in the circulation path  38  in the developer transport direction therein, carrying over of the developer can occur in addition to the leakage of developer. Carrying over of developer means the phenomenon that the developer fails to leave the development sleeve  34   a  in the portion where the development sleeve  34   a  faces the circulation path  38  and is again transported to the development range A. The toner concentration of such carried-over developer is lower. Therefore, when the carrying over of developer occurs, a necessary amount of toner cannot be supplied to the development range A, thus decreasing the image density. 
     It is to be noted that both leakage of toner and carrying over of developer tend to occur when the amount of developer is relatively large around the downstream end in the circulation path  38  in the developer transport direction. That is, leakage of toner and carrying over of developer do not occur when the amount of developer is relatively small and the height thereof is lower around the downstream end in the circulation path  38  in the developer transport direction. 
     Next, prevention of the leakage of toner and the carrying over of developer in the development device  3  is described below. The amount of developer can be kept relatively small around the downstream end in the circulation path  38  in the developer transport direction with the following features: (a) increasing the force to transport the developer around the downstream end in the circulation path  38  in the developer transport direction or (b) setting the area and the position of the bring-up port  41  properly. 
     Eighth Embodiment 
     An eighth embodiment including the feature (a) is described below. The feature (a) can be added to the development device  3  according to any one of the above-described first through seventh embodiments. 
     To achieve the feature (a), a lead angle of the circulation screw  40  transporting the developer in the circulation path  38  should be set properly. 
     A lead angle β of a screw  80  usable as the supply screw  39  and the circulation screw  40  is described below with reference to  FIG. 39 . Referring to  FIG. 39 , the lead angle β is an angle formed by a virtual plane  80   c  and a face of a bladed screw spiral  80   b  fixed to a screw shaft  80   a . The lead angle β decreases as the inclination of the blade screw spiral  80   b  approaches an inclination perpendicular to the screw shaft  80   a . The lead angle β can be expressed using formula 3 shown below. 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       [ 
                       
                         B 
                         
                           2 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           A 
                         
                       
                       ] 
                     
                   
                 
               
               
                 3 
               
             
           
         
       
     
     wherein A represents the diameter of the screw  80 , and B represents the screw pitch thereof. 
       FIG. 40  is a graph illustrating the relation between the lead angle β and the transport velocity of the screw  80 . 
       FIG. 40  shows the transport velocity in the axial direction of the screw  80  measured in an experiment in which the diameter and the rotational velocity of the screw  80  were set to 14 mm and 800 rpm, respectively. 
     As shown in  FIG. 40 , the transport velocity can be increased by setting the lead angle β to an angle close to 45°. Thus, it is preferred to set the lead angle of a portion of the circulation screw  40  around the downstream end in the circulation path  38  in the developer transport direction an angle of about 45°. Alternatively, the transport velocity of the developer can be increased by increasing the rotationally number of the circulation screw  40 . However, when the rotational number of the screw is higher, heat is generated due to friction between the screw and the bearing portion. Because such heat might cause toner in the developer to coagulate and be solidified, it is preferred that the rotational number of the screw be as small as possible. Therefore, in the development device  3 , the portion of the circulation screw  40  around the downstream end in the circulation path  38  in the developer transport direction has a lead angle of about 45° and then the rotational number thereof is set to a necessary number. As an example, the circulation screw  40  rotates at a rotational number of 800 rpm in the eighth embodiment. 
     Additionally, it is preferred that the circulation screw  40  should have such a configuration to apply an upward force to the developer at the position where the bring-up port  41  is disposed in the circulation path  38 . More specifically, it is preferred that the lead angle of a portion of the circulation screw  40  corresponding to the bring-up port  41  in the circulation path  38  be close to an angle of 60°, for example, within a range from 45° to 70°, which is described in further detail below with reference to  FIG. 41 . 
       FIG. 41  illustrates a flow of the developer around the downstream end in the circulation path  38  in the developer transport direction. 
     In  FIG. 41 , the portion around the downstream end in the circulation path  38  includes the bring-up portion  41   a  and an upstream portion  41   b  positioned upstream from the bring-up portion  41   a  in the developer transport direction. In  FIG. 41 , arrow W indicates the distance between the bring-up port  41  and the development sleeve  34   a  in the axial direction. 
     As shown in  FIG. 41 , the developer is transported to the left in  FIG. 41  to the upstream portion  41   b  positioned upstream from the bring-up portion  41   a , and then transported upward in the bring-up portion  41   a.    
     Therefore, the circulation screw  40  needs different functions in the bring-up portion  41   a  and the upstream portion  41   b.    
     The portion of the circulation screw  40  corresponding to the upstream portion  41   b  needs to transport the developer in a horizontal direction efficiently, and thus the lead angle of that portion is set to an angle around 45° 
     By contrast, because the portion of the circulation screw  40  corresponding to the bring-up portion  41   a  needs to apply upward force to the developer, it is preferable that an inclined paddle is provided in that portion. Alternatively, the lead angle of that portion is set to an angle greater than 45°. 
       FIG. 42  illustrates the screw  80  having a smaller lead angle, and  FIG. 43  illustrates the screw  80  having a larger lead angle. In  FIGS. 42 and 43 , arrow E represents the developer transport direction. 
     The bladed screw spiral  80   b  of the screw  80  applies a vertical repulsion indicated by arrow f shown in  FIGS. 42 and 43 , which is vertical to the face of the screw spiral  80   b , to the developer. The vertical repulsion f is formed by a component f 1  perpendicular to the axial direction and a component f 2  in the axial direction. 
     That is, the vertical repulsion f is determined by the lead angle β (shown in  FIG. 39 ). When the lead angle is smaller, the component f 2  in the axial direction is larger in the vertical repulsion f as shown in  FIG. 42 . As the lead angle increases, the component f 2  in the axial direction decreases, and simultaneously, the component f 1  in the direction perpendicular to the axial direction increases as shown in  FIG. 43 . 
     Therefore, upward force applied to the developer can be increased by increasing the lead angle of the circulation screw  40  in the portion corresponding to the bring-up portion  41   a . However, when the lead angle is excessively large, the component f 1  in the direction perpendicular to the axial direction is dominant in the vertical repulsion f, which is not desirable because the amount of developer entering the bring-up portion  41   a  decreases in this state. If the circulation screw  40  does not have a force to transport the developer in the axial direction at all in the bring-up portion  41   a , the developer accumulates around a boundary between the bring-up portion  41   a  and the upstream portion  41   b  even if the circulation screw  40  tries to transport the developer in the axial direction in the upstream portion  41   b . As a result, the amount of developer entering the bring-up portion  41   a  decreases, which reduces the efficiency in transporting the developer upward in the bring-up portion  41   a.    
       FIG. 44  illustrates a relation between the lead angle of the circulation screw  40  in the bring-up portion  41   a  and the amount of the developer transported upward in the bring-up portion  41   a  when the transportation of the developer in the development device  3  is in equilibrium. 
     In the example shown in  FIG. 44 , the amount of the developer transported upward in the bring-up portion  41   a  is greatest when the lead angle is about 60°. 
     From the graph shown in  FIG. 44 , it can be known that the developer can be transported efficiently from the circulation path  38  through the bring-up portion  41   a  to the supply path  37  by setting the lead angle of the screw blade  80   b  of the circulation screw  40  in the bring-up portion to an angle around 60°. It is to be noted that, instead of setting the lead angle of the screw blade to about 60°, to efficiently transport the developer upward, a paddle may be provided in the portion corresponding to the bring-up portion  41   a  and the lead angle of the paddle may be set to about 60°. 
     In the eighth embodiment, the lead angle of the circulation screw  40  is set to about 45° in the portion around the downstream end in the circulation path  38 , which means, for example, one third of the circulation path  38  divided in the developer transport direction, disposed on the downstream side in the developer transport direction, and set to an angle greater than 45° in the portion upstream from the portion around the downstream end in the circulation path  38 . Additionally, the lead angle of the circulation screw  40  is set to an angle about 60° in the portion corresponding to the bring-up portion  41   a  inside the portion around the downstream end in the circulation path  38 . 
     With this configuration, because the force to transport the developer can be increased around the downstream end in the circulation path  38  in the developer transport direction, and simultaneously the developer can be transported efficiently from the circulation path  38  through the bring-up portion  41   a  to the supply path  37 , the amount of developer accumulating in the portion around the downstream end in the circulation path  38  can be reduced, preventing or inhibiting the leakage of the developer and the carrying over of the developer. 
     Ninth Embodiment 
     A development device  3 G according to a ninth embodiment including the feature (b) is described below. 
     The feature (b) can be added to the development device  3  according to any one of the above-described first through eighth embodiments. 
     Important factors regarding the feature (b) are the shape of the bring-up port  41  and the position thereof relative to the development sleeve  34   a  on a virtual plane perpendicular to the axial direction of the development sleeve  34   a  in the bring-up port  41 . 
       FIG. 45  illustrates a cross section of the development device  3 G on the virtual plane perpendicular to the axial direction of the development sleeve  34   a  in the bring-up port  41 . It is to be noted that, at the position in the axial direction where the bring-up port  41  is disposed, the developer is not supplied to the development sleeve  34   a  nor collected therefrom. Therefore, a portion  65  divides the space above the barrier  43 , between the supply screw  39  and the development sleeve  34   a , and a partition  66  divides the space between the circulation screw  40  and the development sleeve  34   a.    
     In the development device  3 G shown in  FIG. 45 , the bring-up port  41  is formed in the partition  36  on the side away from the development sleeve  34   a  on the virtual plane. 
     Additionally, in the development device  3 G, the circulation screw  40  rotates counterclockwise (in a direction indicated by arrow J) in  FIG. 45 . Thus, a bladed screw spiral  40   b  moves upward on the right of a screw shaft  40   a  in  FIG. 45  and downward on the left of the screw shaft  40   a  in  FIG. 45 . Because the surface of the developer in the circulation path  38  is higher on the right of the screw shaft  40   a  in FIG.  45  accordingly, the bring-up port  41  is formed in an upper portion on the right of the screw shaft  40   a  in  FIG. 45  in the circulation path  38  to bring up the developer to the supply path  37  from the side where the surface of the developer is higher. The efficiency in transporting the developer upward in the bring-up portion  41   a  can be enhanced by bringing up the developer from the side where the surface of the developer is higher as in the present embodiment. 
       FIG. 46  illustrates a cross section of a development device  3 Z 7  according to a comparative example 7 in which a bring-up port  41 Z is closer to the development sleeve  34 Z 1  on the virtual plate than in the ninth embodiment shown in  FIG. 45 . 
     In the development device  3 G and  3 Z 7  respectively shown in  FIGS. 45 and 46 , a gap is present on a side closer to the development sleeve  34   a Z 1  in the bring-up port  41 Z unless the developer clogs the bring-up port  41 Z. Therefore, the developer that has been sent to the supply path  37 Z falls through the gap in the bring-up port  41 Z to the circulation path  38 Z as indicated by arrow K shown in  FIG. 46 . Additionally, because the circulation screw  40 Z rotates in the direction identical to the rotational direction of the circulation screw  40  shown in  FIG. 45 , the surface of the developer in the circulation path  38 Z is higher on the side away from the development sleeve  34 Z 1  than on the side closer to it. Therefore, in the development device  3 Z 7  shown in  FIG. 46 , to bring up the developer from the circulation path  38 Z to the supply path  37 Z, a greater amount of developer should be present in the bring-up port  41 Z than that in the development device  3 G shown in  FIG. 45  because the developer should clog the bring-up port  41 Z. Thus, although the surfaces of the developer in  FIGS. 45 and 46  are at a similar highest, in the comparative example 7, a greater amount of developer is necessary in the bring-up portion to fill the bring-up port  41 Z with the developer close to the development sleeve  34   a Z 1  because the developer falls from the supply path  37 Z through the gap in the bring-up port  41 Z as indicated by arrow K shown in  FIG. 46 . However, when the developer accumulates in the bring-up portion  41   a , the developer accumulates also in the portion  41   b , thus increasing the possibility of occurrence of carrying over and leakage of developer. 
     By contrast, in the ninth embodiment, because the bring-up port  41  is disposed away from the development sleeve  34   a , the developer can be brought up from the circulation path  38  to the supply path  37  even when the amount of the developer is smaller than that in the comparative example 7. Accordingly, the amount of developer present in the upstream portion  41   b  upstream from the bring-up portion  41   a  can be smaller, thus inhibiting the carrying over and leakage of developer. Therefore, it is preferred that the bring-up port  41  is disposed away from the development sleeve  34   a  on the vertical plane perpendicular to the axial direction of the development sleeve  34   a.    
     Next, the shape of the bring-up port is described below with reference to  FIG. 47 . 
       FIG. 47  schematically illustrates the portion around the downstream end in the circulation path  38  in the development device  3 G shown in  FIG. 45  viewed from above. 
     In  FIG. 47 , a reference character γ represents an area where the developer in the circulation path  38  contacts a lower side of the partition  36 . The developer can be brought up from the circulation path  38  to the supply path  37  only when the bring-up port  41  is clogged with the developer, and the developer contacts the partition  36  around the bring-up port  41  in that state. In other words, the bring-up port  41  should be formed in a portion inside the area γ where the developer contacts the lower side of the partition  36  in the circulation path  38 . 
     The shape of the bring-up port  41  is not limited to the shape shown in  FIG. 47  but can be triangular as shown in  FIG. 48 , trapeziform as shown in  FIG. 49 , or rounded as shown in  FIG. 50 , for example. Although the bring-up port  41  can have any shape, the bring-up port  41  preferably has such a shape that its length in the axial direction of the development sleeve  34   a  increases toward the side away from the development sleeve  34   a . It is preferred that the bring-up port  41  be formed on the side away from the development sleeve  34   a  in the partition  36  as described above. 
     Additionally, it is preferred that the area of the bring-up port  41  be larger than that of the cross section of the circulation screw  40 , which is described later with reference to  FIG. 59 . It is to be noted that the area of the cross section of the screw means the area of a circle formed by external shape of the blade on a cross section perpendicular to the screw shaft of the screw. 
     Next, dispersion of fresh toner supplied to the development device  3  is described below. 
       FIG. 51  illustrates a toner supply position T where fresh toner is supplied to the development device  3  shown in  FIG. 38 . The fresh toner is supplied on the upstream side in the circulation path  38  in the developer transport direction. While mixed with the existing developer, the supplied toner is transported sequentially from the circulation path  38  through the bring-up port  41  and the supply path  37  up to the surface of the development sleeve  34   a.    
     To compensate for the amount of consumed toner, 0.05 g of toner is supplied to the development device  3  in each supply operation. For example, when 0.3 g of toner is to be supplied, the supply operation is repeated six times intermittently. The toner supplied in each supply operation (0.05 g) should be dispersed in the developer in the longitudinal direction (axial direction) of the development sleeve  34   a  while transported. 
     Dispersion of supplied toner while the circulation screw  40  transports the developer is to be described later. 
     Dispersing the supplied toner in the bring-up portion  41   a  is described below. 
     To disperse the supplied toner in the longitudinal direction, the path through which the supplied toner is transported in the bring-up portion  41   a  may be divided. For example, as shown in  FIG. 52 , the bring-up port  41  may be divided into a first port  411  and a second port  412 . When the bring-up port  41  is thus divided, the developer flows through two different paths in the bring-up portion  41   a  as shown in  FIG. 53 . With this configuration, even when developer whose toner concentration is higher enters the bring-up portion  41   a  locally, the toner concentration of the developer that has passed through the bring-up portion  41   a  can be equalized because the developer is divided into two different paths. 
     It is to be noted that, when the bring-up port  41  is divided, the opening area of the second port  412  positioned upstream from the first port  411  is preferably smaller than that of the first port  411  as shown in  FIG. 52 . If the opening area of the second port  412  is larger, most of the developer passes through the second port  412 , and thus dispersion effect is reduced. Further, when the total opening area of two divided ports  411  and  412  is greater than the cross section of the circulation screw  40 , clogging of the developer can be prevented. 
     It is to be noted that the dispersibility of the supplied toner can be enhanced also when the bring-up port  41  has such a shape that its width in the direction perpendicular to the axial direction of the development sleeve  34   a  is reduced toward upstream in the developer transport direction as in  FIGS. 48 and 49  because, similarly to when the bring-up port  41  is divided, time lag is caused in bringing up the developer between the upstream side and the downstream side in the developer transport direction in the circulation path  38 . 
       FIG. 54  is a graph illustrating the relation between the shape of the bring-up port  41  and dispersion coefficient of toner, which is a coefficient D used in a transport and dispersion equation of developer expressed by formula 3 shown below. 
     In formula 4, C represents the toner concentration, and the left part represents changes in the toner concentration per unit time. In the right part, a first term is a transport term concerned with movement of the developer in the axial direction and a second term is a dispersion term concerned with dispersion of toner in the developer. When the dispersion coefficient D in the above-described formula 4 is larger, dispersion of toner in the developer is enhanced, that is, the toner can be better mixed with the existing developer, in the longitudinal direction. 
     The toner concentration is measured immediately after fresh toner is supplied and after the developer is brought up from the circulation path  38  to the supply path  37 , and the coefficients D and u in formula 4 are calculated based on the distance by which the developer is transported (hereinafter “transport distance”) and the time during which the developer is transported (hereinafter transport time). 
       FIG. 54  illustrates the results of calculation of the dispersion coefficient. In  FIG. 4 , “rectangular” represents the result when the bring-up port  41  has the shape shown in  FIG. 47 . Similarly, “triangular” and “divided” represent the results when the bring-up port  41  has the shapes shown in  FIGS. 48 and 52 , respectively. 
     Thus, dispersibility of the supplied toner is better when the bring-up port  41  has such a shape as shown in  FIG. 48 ,  49 ,  50 , or  52  than when the bring-up port  41  is rectangular as shown in  FIG. 47 . 
     Tenth Embodiment 
     A tenth embodiment regarding transportation of the developer is described below. The feature of the tenth embodiment is applicable to the development device  3  according to any one of the above-described first through ninth embodiments. 
     The flow of the developer in the development device  3  is described below. 
     As shown in  FIG. 51 , the developer passes through an upstream end portion  38   a  in the circulation path  38 , a downstream end portion  38   b  in the circulation path  38 , an upstream end portion  37   a  in the supply path  37 , and a downstream end portion  37   b  in the supply path  37  and thus is circulated in the development device  3 , forming a backflow (hereinafter “main backflow”). In addition to the main backflow, a certain amount of developer flows from the supply path  37  through the surface of the development sleeve  34   a  to the circulation path  38  (hereinafter “branched flow”). When the main backflow flows at a similar velocity across the entire development device  3 , the amount of the developer increases toward downstream in the circulation path  38  and increases toward upstream in the supply path  37  in the developer transport direction. Therefore, a surface  32   f  of the developer is oblique in the longitudinal direction as shown in  FIG. 51 .  FIG. 55  illustrates a flow of the developer in a given area Z in the circulation path  38  shown in  FIG. 51 . 
     In the given area Z shown in  FIG. 51 , three different flows of developer, namely, a flow Mu entering the area Z from upstream, a flow Mk going downstream from the area Z, and a flow Ms falling from the surface of the development sleeve  34   a , collected in the area Z, are present. 
     In the development device  3 , when the amount of the developer present in the area Z (hereinafter “circulation path cell”) is in equilibrium, the relation among the three flows of the developer can be expressed by the following formula A1.
 
 Mk=Mu+Ms   (A1)
 
     In other words, the amount of developer entering the circulation path cell Z equals to that of developer going out the circulation path cell Z. 
     Additionally, the amount of developer in the flow Mu is determined by multiplying a cross-section area Su of the developer in an upstream end portion in the circulation path cell Z in the developer transport direction with a transport velocity Vu in the upstream end portion in the circulation path cell Z, which can be expressed by formula A2 shown below.
 
 Mu=Su×Vu   (A2)
 
     Similarly, the amount of developer in the flow Mk is determined by multiplying a cross-section area Sk of the developer in a downstream end portion in the circulation path cell Z in the developer transport direction with a transport velocity Vk in the downstream end portion in the circulation path cell Z, which can be expressed by formula A3 shown below.
 
 Mk=Sk×Vk   (A3)
 
     When the values of the transport velocities Vu and Vk are identical, the cross-section area Sk of the developer in the downstream end portion is larger than the cross-section area Su of the developer in the upstream end portion by an area corresponding to the amount of developer falling from the development sleeve  34   a . When the relation Vk=Vu=V is established, formula A4 shown below can be obtained.
 
 Sk×V=Su×V+Ms   (A4)
 
     The above-described formula A4 can be converted into formula A5 shown below.
 
 Sk=Su+Ms/V   (A5)
 
     That is, the cross-section area Sk of the developer in the downstream end portion is larger than the cross-section area Su of the developer in the upstream end portion by an area corresponding “Ms/V”. 
     Based on the above-described theory, the surface  32   f  of the developer is oblique in the development device  3  in the longitudinal direction. 
     Formula A6 shown below can be obtained from the formulas A1 through A3.
 
 Sk×Vk=Su×Vu+Ms   (A6)
 
     It is to be noted that the cross-section area Sk of the developer in the downstream end portion can be identical to the cross-section area Su of the developer in the upstream end portion (Sk=Su=S), that is, the height of the developer is not oblique but is constant, by configuring the development device  3  so that formula A7 shown below is established.
 
 Vk=Vu+Ms/S   (A7)
 
     The formula A7 means that the transport velocity Vk in the downstream end portion is increased from the transport velocity Vu in the upstream end portion by a value corresponding to “Ms/S”. In other words, the surface  32   f  of the developer in the circulation path  38  can be uniform by increasing the transport velocity toward downstream in the developer transport direction in the circulation path  38 . 
     Although the description above concerns the transport velocity of the developer in the circulation path  38 , the transport velocity in the supply path is determined based on the similar theory. 
     Next, shortage of the developer, which can occur around the downstream end portion  37   b  in the supply path  37  in the developer transport direction, are described below. 
     When the developer is transported at a similar velocity across the entire supply path  37 , the amount of the developer decreases toward downstream in the supply path  37  as shown in  FIG. 51 , and accordingly the amount of the developer tends to be smaller around the downstream end portion  37   b . If the amount of the developer is smaller than a certain amount round the downstream end portion  37   b , the developer cannot be supplied to the development sleeve  34   a  from the supply path  37 . As a result, the developer cannot be supplied to an area H positioned in a right side portion of the development sleeve  34   a  shown in  FIG. 56 , which is called shortage of the developer in this specification. 
     Conditions to prevent the shortage of the developer are described below with reference to  FIG. 57 . 
     As the condition to prevent the shortage of the developer around the downstream end portion  37   b , at least the relation expressed by formula A8 shown below should be established between the amount (Mku) of developer transported from the circulation path  38  to the supply path  37  per unit time and the amount (Mzs) of developer falling from the development sleeve  34   a  to the circulation path  3  per unit time shown in  FIG. 57 .
 
 Mku&gt;Mzs   (A8)
 
     That is, when the transportation of the developer in the development device  3  is in equilibrium, the amount (Mku) of developer transported from the circulation path  38  to the supply path  37  per unit time should be greater than the amount (Mzs) of developer falling from the development sleeve  34   a  to the circulation path  3  per unit time. If the relation expressed by the formula A8 is not established, the developer runs short how fast the transport velocity of the developer in the supply path  37  except the upstream end portion  37   a.    
     To prevent the shortage of developer, the amount of developer transported from the circulation path  38  through the bring-up port  41  to the supply path  37  per unit time should be greater than the amount of developer passing through the development range A per unit time, carried on the development sleeve  34   a.    
     The relation between the lead angle β of the screw and the transport force of the screw is described above with reference to  FIGS. 39 through 44  in relation to the feature (a) to inhibit the leakage and carrying over of the developer. Herein, the relation between the lead angle β of the screw and dispersibility of supplied toner is described below. 
       FIG. 58  is a graph illustrating the relation between the lead angle β of the screw and dispersibility of supplied toner. 
     From the graph shown in  FIG. 58 , it can be known that the dispersibility of supplied toner is better when the lead angle β of the screw is larger. It is to be noted that the dispersibility of supplied toner means the degree by which the supplied toner is dispersed while the developer moves a unit distance in the axial direction of the screw. 
     Regarding the above-described feature (b) to keep the amount of developer relatively small around the downstream end in the circulation path  38 , it is preferred that the bring-up port  41  is disposed away from the development sleeve  34   a  on the vertical plane perpendicular to the axial direction of the development sleeve  34   a  as described above with reference to  FIGS. 45 and 46 . 
     In addition, it is preferred that the distance indicated by arrow W in  FIG. 41  between the bring-up port  41  and the development sleeve  34   a  in the axial direction of the development sleeve  34   a  be as long as possible. 
     As described above with reference to  FIGS. 45 and 46 , the developer falls from the supply path  37  through the gap in the bring-up port  41  to the circulation path  38 Z unless the developer clogs the bring-up port  41 . 
     Therefore, to bring up the developer through the bring-up port  41 , a certain amount of developer should be accumulated in the bring-up portion  41   a  in the circulation path  38 . Because accumulating the developer around the bring-up portion  41   a  can cause carrying over and leakage of developer, such inconveniences can be inhibited when the distance W (shown in  FIG. 41 ) between the bring-up port  41  and the development sleeve  34   a  is relatively long. 
     However, the bring-up port  41  is preferably disposed close to the development sleeve  34   a  in the axial direction to keep the development device  3  relatively compact. Therefore, keeping the development device  3  relatively compact while inhibiting carrying over and leakage of developer cannot be attained by only setting the distance W between the bring-up port  41  and the development sleeve  34   a  properly. 
     To achieve both of these objectives, the opening area of the bring-up port  41  should be larger than the cross section of the circulation screw  40  because the amount of developer transported by the screw is determined by multiplying the cross section in which the developer is movable with the transport velocity as expressed by the above-described formulas A2 and A3. When the opening area of the bring-up port  41  is smaller than the cross section of the circulation screw  40 , the developer moving through the bring-up port  41  needs to move at a velocity higher than the velocity at which the developer moves in an area where the circulation screw  40  gives a transport force to the developer. When the velocity of the developer moving through the bring-up port  41  is higher, a greater pressure is applied around the developer, and this pressure is transmitted to the bring-up portion  41   a  in the circulation path  38 . As a result, the developer is likely to accumulate also around downstream end portion  38   b  (shown in  FIG. 51 ) in the circulation path  38  in the developer transport direction, that is, the bring-up portion  41   a  and the upstream portion  41   b  (shown in  FIG. 41 ). 
     In other words, when the bring-up port  41  serving as a first port has an opening area sufficiently large so that the developer packed while the circulation screw  40  makes one rotation can pass the bring-up port  41 , clogging of the bring-up port  41  can be prevented, and stress to the developer can be reduced. 
       FIG. 59  is a graph illustrating the relation between the opening area of the bring-up port  41  and the amount of transported developer. 
     In  FIG. 59 , the graph of “OPENING AREA SMALLER” is when the opening area of the bring-up port  41  is 90% of the cross section of the screw, and the graph of “OPENING AREA LARGER” is when the opening area of the bring-up port  41  is 220% of the cross section of the screw. 
     As shown in  FIG. 59 , even when the rotational number (transport force of the circulation screw  40 ) is identical, the mount of transported developer is larger when the opening area of the bring-up port  41  is larger. In other words, even when the pressure of the developer in the bring-up portion  41   a  is smaller, the amount of transported developer can be increased to a required amount by increasing the opening area of the bring-up port  41 . Therefore, clogging of the developer in the bring-up portion  41   a  and the upstream portion  41   b  can be inhibited. 
     Dispersion of toner around the upstream end portion  38   a  in the circulation path  38  is described below. 
     The amount of developer contained in the casing  33  is smaller in the present embodiment than in typical development devices because the development device  3  according to the present embodiment is relatively compact. An enhanced ability to disperse toner is required when the amount of developer is thus smaller. 
     Table 3 shows specifications of development devices A and B whose capacity of containing developer in the development containing part is different. The development devices A and B consume an identical amount of toner when the number of output sheets per unit time is identical. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Development 
                 Development 
               
               
                   
                 device A 
                 device B 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Capacity of developer in developer 
                   90 g 
                  270 g 
               
               
                 containing part 
               
               
                 Toner concentration 
                   8% 
                   8% 
               
               
                 Amount of toner in developer containing part 
                 7.2% 
                 21.6% 
               
               
                 Toner consumption 
                 0.44 g 
                 0.44 g 
               
               
                 Ratio of toner consumption to amount of 
                 6.10 g 
                 2.04 g 
               
               
                 toner in developer containing part 
               
               
                   
               
            
           
         
       
     
     It is to be noted that in the table 3, the toner concentration is the amount of toner divided by the amount of developer, and the toner consumptions is the amount of toner consumed when A4-size solid images are recorded on sheets. 
     Referring to the table 3, the total amounts of developer contained in the development containing parts of the development devices A and B are 90 g and 270 g, respectively. Although the development devices A and B consume an identical amount of toner to form an identical image when the number of output sheets per unit time is identical, the ratio of toner consumption to the total amount of toner in the development containing part is different between the development devices A and B. For example, to form A4 full-size solid images, the ratio of toner consumption to the total amount of toner in the development containing part is 6% in the development device A and 2% in the development device B. 
     After images are output, the identical amount of toner to the toner consumption should be supplied to the developer containing part for subsequent image formation. Therefore, the amounts of toner supplied to the development devices A and B are respectively equivalent to 6% and 2% of the total amount of toner in the development containing part. Because the ratio of the amount of supplied toner to the total amount of toner is greater in the development device A than in the development device B, the development device A should have a higher ability to disperse the supplied toner in the developer. 
     Increasing the ability to disperse supplied toner in developer in the relatively compact development devise  3  is described below. 
     In the present embodiment, the toner supply position T is disposed in the upstream end portion  38   a  in the circulation path  38  as shown in  FIG. 51 . It is to be noted that, alternatively, the toner supply position may be disposed in a downstream end portion of the supply path  37 , downstream from the position where the developer is supplied to the downstream end of the development sleeve  34   a  in the developer transport direction in the supply path  38 . 
     To enhance the ability to disperse the supplied toner into the developer, the following processes (c), (d), and (e) should be performed. 
     (c) Dispersing the supplied toner in the longitudinal direction of the development sleeve  34   a.    
     (d) Merging the dispersed toner into the developer. 
     (e) Bring the toner into contact with carrier particles to charge the toner electrically. 
     The process (c) should be performed around the upstream end portion  38   a  shown in  FIG. 51  in the circulation path  38 . 
     To perform the process (c) while the circulation screw  40  transports the developer, the toner should overstride the bladed screw spiral  40   b  as indicated by arrows G in  FIG. 60 . It is to be noted that, although  FIG. 60  illustrates movement of toner overriding the screw spiral  40   b  in the direction opposite the develop transport direction indicated by arrow E shown in  FIG. 60 , this does not means that the toner moves in the direction opposite the develop transport direction.  FIG. 60  means that, although a certain amount of toner is transported at a velocity identical to the transport velocity of the developer, a certain amount of toner is delayed for the distance equivalent to the pitch or pitches of the screw spiral  40   b , dropping astern the screw spiral  40   b.    
     Not only the supplied toner, but also a certain amount of developer is delayed, dropping astern the pitch of the screw spiral  40   b . Consequently, the transport velocity of the developer is slowed around the upstream end portion  38   a  in the circulation path  38 . 
       FIG. 61  illustrates a configuration to enhance dispersibility of the supplied toner. 
     A circulation screw  40 - 1  shown in  FIG. 61  includes paddles  401  provided in the pitches of the screw spiral  40   b . The paddles  401  can flick the developer, which is transported by the circulation screw  40 - 1  in the axial direction, in the direction perpendicular to the axial direction. The velocity of developer flicked by the paddles  401  in the axial direction is substantially zero, and the circulation screw  40 - 1  rotates while the paddles  401  flick the developer. Therefore, when the flicked developer returns, the developer is delayed for a distance equivalent to the pitch of the screw spiral  40   b . Based on the above-described theory, the supplied toner can be dispersed in the longitudinal direction. It is to be noted that providing the paddles  401  can decrease the transport velocity (e.g., transport ability of the circulation screw) in the axial direction. Therefore, enhancing the dispersibility around the upstream end portion  38   a  as well as slowing the transport velocity in that portion can be attained by providing the paddles  401  in the portion corresponding to the vicinity of the upstream end portion  38   a.    
     It is to be noted that, when the paddles  401  extending in the direction of the screw shaft  40   a  are provided between the pitches of the screw spiral  40   b  as shown in  FIG. 61 , the length of the paddle  401  in the axial direction per unit length in the screw shaft may be increased toward upstream in the transport direction of the circulation screw  40  because the decrease in the developer transport ability as well as the increase in the dispersibility are more significant as the size of the paddle increases. More specifically, when the pitch of the screw spiral  40   b  is constant or substantially constant in the portion where the paddles  401  are provided, the length of each paddle  401  in the axial direction is increased toward upstream in the transport direction of the circulation screw  40 . By contrast, when the pitch of the screw spiral  40   b  is not constant in the portion where the paddles  401  are provided, the length of the paddle(s)  401  in a given length in the axial direction is increased toward upstream in the transport direction of the circulation screw  40 . 
     Alternatively, to enhance the dispersibility of the supplied toner, a screw, such as the screw  80  shown in  FIG. 43 , whose lead angle is greater may be used. 
     As described above with reference to  FIG. 43 , when the lead angle of the screw is greater, that is, the screw spiral  80   b  is more horizontal relative to the screw shaft  80   b , upward force applied to the developer can be increased, that is, the developer can overstride the screw spiral  80   b  more easily. As a result, dispersibility of toner in the longitudinal direction can be enhanced. Further, as shown in  FIG. 40 , when the lead angle is greater than 45°, the greater the lead angle, the lower transport velocity is. 
     Therefore, in the present embodiment, the lead angle of the circulation screw  40  is set to an angle about 65° in the portion corresponding to the upstream end portion  38   a , decreases toward downstream in the developer transport direction, and is set to an angle of about 45° in the vicinity of the downstream end in the circulation path  38  in the developer transport direction described in the eighth embodiment. 
     It is to be noted that, alternatively, to enhance the dispersibility of the supplied toner, a circulation screw  40 - 2  shown in  FIG. 62  in which a part of a screw spiral  40   b   1  is cut off, forming a cutout  40   d , may be used. Since the supplied toner moves in an upper portion, that is, vicinity of surface  32   f  (shown in  FIG. 51 ), of the developer layer in the development device  3 , also the circulation screw  40 - 2  shown in  FIG. 62  can delay the developer as described above with reference to  FIG. 60 . To slow the transport velocity of developer further from the velocity attained by the configuration shown in  FIG. 62 , a circulation screw  40 - 3  shown in  FIG. 63  may be used. In the circulation screw  40 - 3 , a part of a screw spiral  40   b   2  is cut off to an extent that a cutout  40   d   1  extends to a center portion of the screw spiral  40   b   2 . 
     It is to be noted that, when the cutout  40   d  is formed in a portion of the screw spiral  40   b  downstream from the upstream end portion  38   a  of the circulation path  38  in the developer transport direction in addition to the upstream end portion  38   a , the area of the cutout  40   d  in the upstream end portion  38   a  may be larger than that in the portion downstream from the upstream end portion  38   a  because the decrease in the developer transport ability as well as the increase in the dispersibility are more significant as the size of the paddle increases. Alternatively, the cutout  40   d  may be formed only in a portion of the screw spiral  40   b  in the upstream end portion  38   a  in the transport direction of the circulation screw  40 . 
     Thus, the development device  3  according to the present embodiment should have such a configuration that the transport velocity can be slowed around the upstream end portion  38   a  while the supplied toner can be dispersed in the longitudinal direction. 
       FIG. 64  is a graph in which the toner dispersibility and the transport velocity are compared among the circulation screw  40  shown in  FIG. 60 , for example, the circulation screw  40 - 1  with paddles  401  shown in  FIG. 61 , and the circulation screw  40 - 2  shown in  FIG. 62 . In  FIG. 64 , the circulation screws  40 ,  40 - 1 , and  40 - 2  are represented by “NORMAL”, “PADDLE”, and “WT OUT”, respectively. 
     All three types of the circulation screws used to obtain the graph shown in  FIG. 64  have a lead angle of 35.5°, and the configurations thereof (e.g., diameter, pitch, and rotational number, etc.) are similar except the paddles  401  and the cutout  40   d . As shown in the graph shown in  FIG. 64 , when the paddles are provided or the cutout is formed in the screw spiral, the transport velocity is slowed, and simultaneously dispersibility of toner is enhanced. The dispersibility of supplied toner means the degree by which the supplied toner is dispersed while the developer moves a unit distance in the axial direction of the screw. 
     As described above, in the present embodiment, because the fresh toner is supplied to the upstream end portion  38   a  in the circulation path  38  in the developer transport direction as shown in  FIG. 51 , the supplied toner tends to coagulate in the upstream end portion  38   a . Because, initially, the coagulated toner should be loosened to agitate such coagulated toner while transporting, the toner should be better dispersed in the upstream end portion  38   a  than in the downstream end portion  38   b  in the developer transport direction in the circulation path  38 . Thus, it is preferable to provide the paddles  401 , cutout  40   d , or the like on the upstream end portion of the circulation screw  40  in the developer transport direction. Additionally, it is preferable that the lead angle of the circulation screw  40  be increase toward upstream in the developer transport direction to increase the dispersibility of the toner as described above. 
     Next, above-described (d), merging the dispersed toner into the developer, is described below. 
       FIG. 65  schematically illustrates a state of supplied toner T and the developer  32  around the upstream end portion  38   a  of the circulation path  38 . 
     As indicated by arrow Q shown in  FIG. 65 , the developer  32  that has left the development sleeve  34   a  is collected in the circulation path  38 . The developer  32  that has left the development sleeve  34   a  falls onto the supplied toner T, which moves in the upper portion of the developer layer in the circulation path. Flowing down the developer that has left the development sleeve  34   a  onto the supplied toner T can facilitate mixing the supplied toner T with the developer  32  in the circulation path  38 . 
     The above-described process (e), bring the toner into contact with carrier particles, can be performed in the bring-up portion  41   a . With the configuration described regarding the processes (c) and (d), the toner supplied to the casing  33  is dispersed in the developer contained in the circulation path  38 . In the bring-up portion  41   a , the pressure of the developer is increased to bring the developer upward, and this increased pressure of developer can increase the number of contact between the toner and carrier particles. Therefore, in the configuration of the development device  3  according to the present embodiment, the supplied toner can be dispersed in the developer sufficiently even when the development device  3  is relatively contact, which means that the capacity of the casing  33  to contain the developer is smaller. 
     Next, increasing the amount of developer on the upstream side in the circulation path  38  as well as on the downstream side in the supply path  37  is described below. 
     When the amount of developer moving in and the amount of developer moving out of a given area in the developer transport path per unit time are set, the amount of developer present in that area can be increased, that is, the surface of the developer layer in that area can be raised, by slowing the transport velocity of the developer in that area. 
     Therefore, the amount of developer on the upstream side in the circulation path  38  as well as that on the downstream side in the supply path  37  can be increased by slowing the transport velocity of the developer in those areas. Following effects (f) and (g) are available by increasing the amount of developer on the upstream side in the circulation path  38  as well as that on the downstream side in the supply path  37 . Thus, by slowing the transport velocity of the developer in the circulation path  38  toward upstream in the circulation path  38  in the developer transport direction, and similarly, by increasing the transport velocity of the developer in the supply path  37  toward upstream in the supply path  37  in the developer transport direction, the surface of the developer can be uniform in the circulation path  38  and supply path  37 , respectively. 
     (f) Expanding the life of the developer in the development device. 
     (g) Reducing fluctuations in the toner concentration in the developer in the development device caused by consumption and supply of toner. 
     Regarding the above-described effect (f), the life of the developer can be expanded by reducing the stress given to the developer around the developer regulator  35  even when the development device  3  is relatively compact as described in the first embodiment. Further, since the life of the developer is proportional to the amount of the developer in the development device  3 , increasing the developer capacity of the casing  33  (e.g., developer containing part) can expand the life of the developer accordingly. 
       FIG. 66  illustrates the amount (height) of developer at respective positions in the developer transport direction in the supply path  37  and the circulation path  38  when the transport velocity of the developer is constant across the entire supply path  37  and the entire circulation path  38 . 
     In  FIG. 66 , a vertical axis represents the amount (height) of developer and a horizontal axis represents the position in the supply path  37  and the circulation path  38  in the developer transport direction. 
     The graph shown in  FIG. 66  is obtained when the lead angle of both the supply screw  39  and the circulation screw  40  is 45° and the configurations thereof are uniform. In this case, the surface  32   f  of the developer is oblique in the longitudinal direction as shown in  FIG. 51 . 
     It is to be noted that, because the developer is transported in the opposite directions in the supply path  37  and the circulation path  38 , the right side in the graph of  FIG. 66  is the downstream side in the supply path  37  and the upstream side in the circulation path  38 . 
       FIG. 67  illustrates the amount (height) of developer at respective positions in the developer transport direction in the supply path  37  and the circulation path  38  when the transport velocity of the developer is varied depending on the position in the developer transport direction therein. 
     More specifically, the lead angle of the circulation screw  40  is set to an angle of about 45° in the portion corresponding to the downstream end portion  38   b , gradually increases toward upstream in the developer transport direction, and is set to an angle about 65° in the portion corresponding to the upstream end portion  38   a . Similarly, the lead angle of the circulation screw  39  is set to an angle of about 45° in the portion corresponding to the upstream end portion  37   a , gradually increases toward downstream in the developer transport direction, and is set to an angle about 65° in the portion corresponding to the downstream end portion  37   b.    
     When the graphs shown in  FIGS. 66 and 67  are compared, the amount of developer in a circle R, that is, the upstream end portion in the supply path  37  as well as the downstream end portion in the circulation path  38 , is identical or similar in  FIGS. 66 and 67  due to the conditions to send the developer from the circulation path  38  to the supply path  37  without leakage of developer or carrying over of developer. By contrast, the amount of developer in the downstream end portion in the supply path  37  as well as the upstream end portion in the circulation path  38  is larger in  FIG. 67  than in  FIG. 66 . 
     By changing the configuration of the development device  3  from that described with reference to  FIG. 66  to that described with reference to  FIG. 67 , the developer capacity of the casing  33  can be increased from 70 g to 90 g, and accordingly the life of the developer can be expanded by 30 percent. Further, the ratio of toner consumption to the total amount of toner in the development containing part when A4 full-size solid images are output can be reduced from 7.9 percent to 6.1 percent. Thus, even when the development device  3  is relatively compact, fluctuations in the toner concentration in the developer therein caused by consumption and supply of toner can be reduced, and accordingly fluctuations in image density can be reduced. 
     It is to be noted that the configuration of the developer transport paths in the development device  3  according to the eight through tenth embodiments is suitable for relatively compact development devices using a three-pole magnet roller and a development sleeve having a relatively small diameter as in the first through seventh embodiments. However, the configuration of the developer transport paths according to the eight through tenth embodiments is also applicable to development devices in which the number of magnetic poles each generating a magnetic field sufficient to carry the developer on the development sleeve is more than four as in the comparative example 2 shown in  FIG. 11  in which the number of such magnetic poles is five. In other words, the configuration of the developer transport paths according to the eight through tenth embodiments is also applicable to development devises including the supply path and the circulation path disposed beneath the supply path. More specifically, in the supply path, the developer is supplied onto the development sleeve while transported in the axial direction of the development sleeve. The circulation path receives the developer from the downstream end portion of the supply path in the developer transport direction, and the developer is collected from the development sleeve in the circulation path while transported in the axial direction of the development sleeve in the opposite direction to the developer transport direction in the supply path. Then, the developer reached the downstream end portion of the circulation path is sent to the upstream end portion of the supply path in the developer transport direction. 
     It is to be noted that, in the above-described various embodiments, it is preferable that the amount of developer is set so that, when the circulation screw  40  is stopped, the surface  32   f  (shown in  FIG. 51 ) of the developer layer in the circulation path  38  is lower than the horizontal axis  34   h  (shown in  FIG. 8 ) extending horizontally from the center of rotation  34   p  (shown in  FIG. 8 ) in the area of the circulation path  38  where the development sleeve  34   a  overlaps in the axial direction. In this configuration, carrying over of developer can be reduced due to the development sleeve  34   a  and the weight of developer itself. 
     Eleventh Embodiment 
     Next, a feature of agitating the developer upstream from the developer regulator  35  in the rotation direction of the development sleeve  34   a , which is applicable to the development device  3  according to the first through tenth embodiments, is described below as an eleventh embodiment. 
       FIG. 68  illustrates a cross section of the image forming unit  17  that is a process cartridge to which the eleventh embodiment is applicable. 
     As shown in  FIG. 68 , a development device  3 H, a charging roller  2   a  of the charger  2  (shown in  FIG. 1 ), and a cleaning blade  6   a  of the cleaner  6  (shown in  FIG. 1 ) are provided around the photoconductor  1 . The charging roller  2   a  is disposed contacting the photoconductor  1  or across a gap from the photoconductor  1 . Alternatively, a brush or a scorotron charging member may be employed instead of the charging roller  2   a.    
     The development device  3 H has a similar configuration to that of the first through eleventh embodiments except that a paddle  51  shown in  FIG. 69  is provided. That is, the paddle  51  is applicable to any of the first through eleventh embodiments. 
     Also in the present embodiment, the developer is poured from the supply path  37  down to the surface of development sleeve  34   a  under gravity as indicated by arrow I shown in  FIG. 68 . Therefore, even when the magnetic force of the pre-development pole N 2  that contributes to attracting the developer and adjusting the amount of the developer is weaker, attracting developer to the development sleeve  34   a  and adjusting the amount of developer can be performed reliably. Additionally, to reduce stress to the developer, the magnetic force of the magnet forming the pre-development pole N 2  is weaker compared to that of magnets typically used to form the developer regulation pole for adjusting the amount of developer. Therefore, the magnetic force of the magnetic field that affects the developer within a buffer area  50  that is an area upstream from the developer regulator  35  in the rotational direction of the development sleeve  34   a  is weaker, and thus the developer is kept softly in the buffer area  50 . 
     The developer supplied from the supply path  37  by the supply screw  39  to the buffer area  50  may be wavy due to the pitch of the supply screw  39 , making the amount of the supplied developer uneven. If the magnetic force of the developer regulation pole is as strong as that in typical development devices, a certain degree of pressure is applied to the developer in the buffer area, and the pressure equalizes the amount of the supplied developer. 
     Additionally, the developer may include loosely coagulated toner particles and/or carrier particles. If such coagulations clog the regulation gap, image failure in which toner is partly absent creating while lines or the like occur. If the pressure given to the developer in the buffer area is relatively strong, the pressure can dissolve such loose coagulations, thus preventing clogging of the regulation gap. 
     However, in the present embodiment, because the magnetic force of the magnetic field affecting the developer in the buffer area  50  is weaker, the pressure applied to the developer in the buffer area  50  is weaker, and accordingly the agitation effect of the magnetic force is lower. Therefore, if the amount of developer supplied by the supply screw  39  is wavy due to the pitch of the supply screw  39 , the magnetic force is insufficient to eliminate unevenness in the amount of developer carried on the development sleeve  34   a , resulting in unevenness in image density. 
     Further, if a certain amount of toner particles and/or carrier particles coagulates in the developer, such coagulation might clog the regulation gap, producing substandard images including white lines. 
     To prevent this inconvenience, the development device  3 H according to the present embodiment includes the paddle  51  serving as an agitation member as shown in  FIG. 69 .  FIG. 70  is a perspective view of the paddle  51 . 
     Referring to  FIG. 69 , blades  51   b  of the paddle  51  can rotate or swing around a rotation shaft  51   a , thereby agitating the developer in the buffer area  50 . Thus, unevenness in the amount of the supplied developer caused by the pitch of the supply screw  39  can be eliminated, and accordingly image density can be uniform. Even when toner particles and/or carrier particles in the developer coagulate, the paddle  51  can dissolve such coagulations, preventing clogging of the regulation gap. Consequently, substandard images including white lines can be prevented or reduced. 
     It is to be noted that the shape of the paddle  51  is not limited to that shown in  FIG. 69 . For example, the number of the blades  51   b  is not limited to four. The shape of the blades  51   b  may be continuous in the longitudinal direction of the paddle  51  as shown in  FIG. 70  or may be fragmentary, alternatively. 
       FIGS. 71A and 71B  illustrates a configuration of a development device  3 H 1  using a roller member  52  as an agitation member instead of the paddle  51  as a variation of the eleventh embodiment.  FIG. 71A  is a cross-sectional view of the development device  3 H 1 , and  FIG. 71B  is a perspective view of the roller member  52 . Referring to  FIGS. 71A and 71B , also when the roller member  52  is used instead of the paddle  51 , by rotating or swinging a roller  52   b  around a rotation shaft  52   a , the developer in the buffer area  50  can be agitated. Additionally, even when the developer coagulates, the roller member  52  can dissolve such coagulations. 
     It is to be noted that the material of the roller  52   b  is not limited to a specific material. For example, a metal roller or a sponge roller may be used as the roller  52   b.    
       FIGS. 72A and 72B  illustrate a configuration of a development device  3 H 2  using a wire member  53  as an agitation member according to another variation of the eleventh embodiment.  FIG. 72A  is a cross-sectional view of the development device  3 H 2 , and  FIG. 72B  is a perspective view of the wire member  53 . 
     Similarly, by rotating or swinging the wire member  53  around a rotation shaft  53   a , the developer in the buffer area  50  can be agitated. Additionally, even when the developer includes coagulations, the wire member  53  can dissolve such coagulations. 
     Thus, in the eleventh embodiment and variations thereof, even when the supply screw  39  supplies the developer unevenly (including the case in which in developer supply is uneven in the longitudinal direction), the above-described agitation member can equalize the developer and loose coagulations of the developer in the buffer area  50 . Therefore, the developer can be uniform upstream from the regulation gap, and a constant amount of developer can be supplied onto the development sleeve  34   a . Therefore, unevenness in image density and image failure can be prevented or reduced. 
     It is to be noted that, although the developer regulator  35  is disposed above the development sleeve  34   a  in the above-described various embodiments, the developer regulator  35  can be disposed beneath the development sleeve  34   a  when the agitation member is provided as in the eleventh embodiment. 
     Twelfth Embodiment 
     A twelfth embodiment regarding a configuration of the development sleeve is described below. The feature of the twelfth embodiment is applicable to the development device  3  according to any one of the above-described first through eleventh embodiments. 
     If developer is brought up onto the development sleeve from beneath, the surface of the development sleeve should be rough to a such a degree that the developer can be kept thereon also mechanically. In such cases, surface treatment such as blast finishing of the development sleeve is necessary. 
     By contrast, the configuration in which the developer flows down onto the development sleeve  34   a , as indicated by arrow I shown in  FIG. 68 , does not require a force to transport the developer upward. 
     Therefore, the development sleeve  34   a  of the present embodiment has a surface roughness Rz within a range from 1 μm to 8 μm, for example, because this range of surface roughness is sufficient for carrying the developer on the surface of the development sleeve  34   a . The surface roughness Rz within a range from 1 μm to 8 μm can be attained by standard turning and may be attained by processing, such as aluminum extrusion, that does not include removal processing. The surface of the development sleeve  34   a  of the present embodiment may be attained through standard cutting without removal processing. It is to be noted that removal processing herein means processing to produce concavities on the surface, and the development sleeve  34   a  in the present embodiment does not require such surface processing. Standard cutting herein means cutting processing that is performed to make the diameter of the sleeve to a predetermined diameter when blast processing is not performed. 
     Therefore, manufacture of the development sleeve  34   a  can be simpler and easier, and accordingly the cost is lower. 
     In particular, when the diameter of the development sleeve is smaller (e.g., 10 mm) and the surface of the development sleeve should be treated to be able to bring up the developer from beneath, the cost of surface treatment is higher. By contrast, in the present embodiment, although the development sleeve  34   a  has a relatively small diameter, the surface of the development sleeve can be relatively smooth because the development sleeve does not require the force to bring up the developer. Thus, the processing cost of the development sleeve  34   a  can be reduced. 
     It is to be noted that, typical turning can attain a surface roughness of about 0.8 μm, and aluminum extrusion can attain a surface roughness of about 3.2 μm. 
     Additionally, wear of the development sleeve  34   a  can be slower when the surface is relatively smooth, thus expanding the operational life. Additionally, compared with typical development sleeves on which grooves are formed, developer particles can stand on end thereon more uniformly on the surface of the development sleeve  34   a , and accordingly development efficiency can be higher. 
     Thirteenth Embodiment 
     A thirteenth embodiment regarding a driving gear to drive the development sleeve is described below. The feature of the thirteenth embodiment is applicable to the development device  3  according to any one of the above-described first through twelfth embodiments. 
       FIG. 73  schematically illustrates relative positions of the photoconductor  1 , the development sleeve  34   a , and a development gear  34   g  to transmit a driving force to the development sleeve  34   a.    
       FIG. 74  schematically illustrates relative positions of a photoconductor  1 Z, a development sleeve  34   a Z, and a development gear  34   g Z in a comparative development device. 
     When the module of the development gear  34   g Z is larger or the number of its tooth is greater as in the comparative example shown in  FIG. 74 , the development gear  34   g Z has an external diameter larger than that of the development sleeve  34   a Z. Therefore, the development gear  34   g Z cannot be disposed within an area facing the photoconductor  1 Z in the longitudinal direction, thus increasing the size of the comparative development device. 
     By contrast, in the present embodiment, the external diameter of the development gear  34   g  is smaller than that of the development sleeve  34   a  and can be disposed within an area facing the photoconductor  1  in the longitudinal direction without interfering with the photoconductor  1 , thus decreasing the size of the development device  3  in the longitudinal direction. 
     An example of a spur gear applicable to the development device  3  of the present embodiment is described below. 
     A diameter dk of the addendum circle of the spur gear can be obtained by the formula A9.
 
 dk=do+ 2 ·m   (A9)
 
     wherein do represents a pitch diameter (z·m), m represents a module, and z represents the number of tooth. 
     When the external diameter of the development sleeve is 10 mm and the number of tooth as undercut limit is 17, from the formula A9, the diameter dk of the addendum circle can be 10 mm or less when the following condition is satisfied.
 
10=17 ·m+ 2 ·m  
 
 m= 0.53
 
     Therefore, the module should be 0.5 mm. 
     Although reducing the size of the module can increase the number of tooth, the strength of the gear decreases as the module becomes smaller. Accordingly, the gear might fail to transmit the driving force to the development sleeve  34   a  if the gear is extremely small. However, in the present embodiment, the load to the development sleeve  34   a  is reduced in the developer regulation portion, which faces the developer regulator  35 , and accordingly the rotational torque of the development sleeve  34   a  is smaller. Consequently, the size of the module can be as small as 0.5 mm, for example. 
     Thus, the development gear  34   g  can have an external diameter smaller than that of the development sleeve  34   a , thus compactness of the development device  3  can be attained. 
     Fourteenth Embodiment 
     A fourteenth embodiment regarding a preset seal is described below. The feature of the fourteenth embodiment is applicable to the development device  3  according to any one of the above-described first through thirteenth embodiments. 
     When development devices are shipped from factory, it is preferable that the developer is preliminarily set in the developer containing part thereof. Therefore, the development device  3  according to the present embodiment is provided with a preset seal to prevent leakage of developer from the casing  33  (developer containing part) during transportation. 
       FIGS. 75 and 76  are cross sectional views illustrate flow of the developer inside the development device  3 .  FIG. 75  illustrates a cross section of the development device  3  in the direction perpendicular to the axial direction of the development device  3 , and  FIG. 76  is an N-N′ cross section of the development device  3  viewed from the direction indicated by arrow C shown in  FIG. 75 . 
     It is to be noted that reference characters I 1  through I 7  represent the flow of the developer. 
     In a short side direction of the development device  3  perpendicular to its longitudinal direction, as shown in  FIG. 75 , the developer is circulated through the supply path  37 , the surface of the development sleeve  34   a , and the circulation path  38  in that order. 
     By contrast, in the longitudinal direction (axial direction of the development sleeve  34   a ), the developer is circulated from the circulation path  38  through the bring-up port  41 , the supply path  37 , and the falling port  42  again to the circulation path  38  in that order. 
       FIGS. 77A ,  77 B, and  77 C illustrate a first seal member  60   a  and a second seal member  60   b  respectively sealing a first communicating area between the supply path  37  and an area in which the development sleeve  34   a  is disposed and a second communicating area between the circulation path  38  and the area in which the development sleeve  34   a  is disposed. The first seal member  60   a  and the second seal member  60   b  together form a seal member  60  serving as the preset seal. As shown in  FIG. 75 , the developer flows in the first and second communicating areas in the directions indicated by arrows I 1  and I 3 , respectively.  FIG. 77A  is a cross section of the development device  3 , and  FIG. 77B  is a perspective view illustrating a configuration in which the first and second seal members  60   a  and  60   b  are separately pulled out from the development device  3 .  FIG. 77C  illustrates a seal member  60 - 1  including a first seal member  60   a   1  and a second seal member  60   b   1  that are united as a single member to be pulled out simultaneously from the development device  3  as a variation of the preset seal. Thus, a common handle or grip may be formed in the united seal member  60 - 1  for users to remove the first seal member  60   a   1  and the second seal member  60   b   1  together from the development device  3  by pulling the handle or grip. 
     By providing the above described seal member, the developer can be preset in both the supply path  37  and the circulation path  38 , and the development device  3  can be transported reliably without leakage of the developer. 
       FIGS. 78A ,  78 B, and  78 C illustrate a configuration in which a single seal member  60 - 2 , as another variation of the preset seal, seals both the first communicating area between the supply path  37  and the area in which the development sleeve  34   a  is disposed and the second communicating area between the circulation path  38  and the area in which the development sleeve  34   a  is disposed. As shown in  FIG. 75 , the developer flows in the first and second communicating areas in the directions indicated by arrows I 1  and I 3 , respectively.  FIG. 78A  is a cross-sectional view of the development device  3 , and  FIG. 78B  is a perspective view illustrating a configuration of the development device  3  in which the seal member  60 - 2  is pulled out horizontally from the development device  3 .  FIG. 78C  is a perspective view illustrating a configuration of a development device  3 I in which a seal member  60 - 3  is pulled out upward therefrom. 
     It is to be noted that, in the configuration shown in  FIG. 78C , the seal member  60 - 3  is pulled through a seal cleaning member  61  from the development device  3 I to remove the developer adhering to the surface of the seal member  60 - 3 . The seal cleaning member  61  is preferably formed with a material such as foamed polyurethane that has a certain degree of flexibility. In the configurations shown in  FIGS. 78A through 78C , the developer can be preset in both the supply path  37  and the circulation path  38  similarly to the configurations shown in  FIGS. 77A through 77C . 
       FIGS. 79A ,  79 B, and  79 C illustrate a configuration in which two seal members, together forming a preset seal, respectively seal the first communicating area between the supply path  37  and the area in which the development sleeve  34   a  is disposed and a third communicating area between the circulation path  38  and the supply path  37 . The third communicating area includes the bring-up port  41  and the falling port  42  formed in the partition  36  shown in  FIG. 76 . As shown in  FIGS. 75 and 76 , the developer flows in the first communicating area in the direction indicated by arrows I 1  and in the third communicating area in the directions indicated by arrows I 5  and I 7 , respectively.  FIG. 79A  is a cross section of the development device  3 , and  FIG. 79B  illustrates a configuration in which a first seal member  60   a  and a third seal member  60   c  (forming a seal member  60 - 4 ) are separately pulled out from the development device  3 . 
       FIG. 79C  is a perspective view illustrating a seal member  60 - 5  that includes a united first seal member  60   a   2  and a third seal member  60   c   1  so that two seal members can be pulled out simultaneously from the development device  3 . Thus, a common handle or grip may be formed in the united seal member  60 - 5  for users to remove the first seal member  60   a   2  and the third seal member  60   c   1  together from the development device  3  by pulling the handle or grip. 
     In the configurations shown in  FIGS. 79A through 79C , the developer can be preset in the supply path  37 . 
       FIGS. 80A ,  80 B, and  80 C illustrate a configuration in which a single seal member, as another variation of the preset seal, seals both the first communicating area between the supply path  37  and the area in which the development sleeve  34   a  is disposed and the third communicating area between the circulation path  38  and the supply path  37  (the bring-up port  41  and the falling port  42  formed in the partition  36 ). Similarly, the developer flows in the first communicating area in the direction indicated by arrows I 1  and in the third communicating area in the directions indicated by arrows I 5  and I 7 , respectively.  FIG. 80A  is a cross section of the development device  3 , and  FIG. 80B  is a perspective view illustrating a configuration of the development device  3  in which a seal member  60 - 6  is pulled out horizontally from the development device  3 .  FIG. 80C  is a perspective view illustrating a configuration of the development device  3 I in which a seal member  60 - 7  is pulled out upward therefrom. 
     It is to be noted that, in the configuration shown in  FIG. 80C , the seal member  60 - 3  is pulled through the seal cleaning member  61  from the development device  3 I to remove the developer adhering to the surface of the seal member  60 - 3  similarly to the configuration shown in  FIG. 78C . In the configurations shown in  FIGS. 80A through 80C , the developer can be preset in the supply path  37  similarly to the configurations shown in  FIGS. 79A through 79C . 
       FIGS. 81A ,  81 B, and  81 C illustrate a configuration in which two seal members, together forming a preset seal, respectively seal the second communicating area between the circulation path  38  and the area in which the development sleeve  34   a  is disposed, and the third communicating area between the circulation path  38  and the supply path  37  (the bring-up port  41  and the falling port  42  formed in the partition  36  shown in  FIG. 76 ). As shown in  FIGS. 75 and 76 , the developer flows in the second communicating area in the direction indicated by arrows I 3  and in the third communicating area in the directions indicated by arrows I 5  and I 7 , respectively.  FIG. 81A  is a cross section of the development device  3 , and  FIG. 81B  illustrates a configuration in which a second seal member  60   b  and a third seal member  60   c  (forming a seal member  60 - 8 ) are separately pulled out from the development device  3 . 
       FIG. 81C  is a perspective view illustrating a second seal member  60   b   2  and a third seal member  60   c   2  together forming a united single seal member  60 - 9  so that two seal members  60   b   2  and  60   c   2  can be pulled out simultaneously from the development device  3 . Thus, a common handle or grip may be formed in the united seal member  60 - 9  for users to remove the second seal member  60   b   2  and the third seal member  60   c   2  together from the development device  3  by pulling the handle or grip. 
     In the configurations shown in  FIGS. 81A through 81C , the developer can be preset in the circulation path  38 . 
       FIGS. 82A ,  82 B, and  82 C illustrate a configuration in which a single seal member, as another variation of the preset seal, seals both the second communicating area between the circulation path  38  and the area in which the development sleeve  34   a  is disposed, and the third communicating area between the circulation path  38  and the supply path  37  (the bring-up port  41  and the falling port  42  formed in the partition  36  shown in  FIG. 76 ). Similarly, the developer flows in the second communicating area in the direction indicated by arrows I 3  and in the third communicating area in the directions indicated by arrows I 5  and I 7 , respectively.  FIG. 82A  is a cross section of the development device  3 , and  FIG. 82B  is a perspective view illustrating a configuration of the development device  3  in which a seal member  60 - 10  is pulled out horizontally from the development device  3 .  FIG. 82C  is a perspective view illustrating a configuration of the development device  3 I in which a seal member  60 - 11  is pulled out upward therefrom. 
     The seal member  60 - 11  is pulled through the seal cleaning member  61  from the development device  3 I to remove the developer adhering to the surface of the seal member  60 - 11  similarly to the configurations shown in  FIGS. 78C and 80C . In the configurations shown in  FIGS. 82A through 82C , the developer can be preset in the supply path  38  similarly to the configurations shown in  FIGS. 81A through 81C . 
     It is to be noted that, although the configurations shown in  FIGS. 77A through 82C  concern searing the development device with the sealing member, the sealing member described above may be used in process cartridges. 
     In addition, although the seal cleaning member  61  is used only when the seal member is pulled out upward from the development device  3  in the description above, it is preferable that the seal cleaning member  61  is provided in the configuration in which the seal member is pulled out horizontally. 
     Thus, by sealing the space in which the development sleeve  34   a  is provided and the third communication area between the supply path  37  and the circulation path  38 , leakage of the developer can be prevented during transportation even when the developer is preset therein. When the development device is used as a replacement part, because package thereof can be kept clean, users do not have a feeling of discomfort. 
     Additionally, scattering of the developer inside the image forming apparatus can be prevented, and accordingly image failure and malfunction of the apparatus thereby can be prevented. 
     In the above described embodiment and variations thereof, the seal member  60  is thermally welded to the casing forming an edge portion of the communicating area to be sealed in the longitudinal direction, thus sealing the communicating area. The seal member  60  is folded on a first side in the longitudinal direction, opposite a second side from which the seal member  60  is pulled out from the development device  3 . The seal member  60  is removed out from the development device  3  by pulling a second end portion of the seal member  60  that is not welded to the casing and is disposed on the opposite side of the welded side across the folded portion. By pulling the folded seal member  60 , not the seal member  60  is removed entirely at once, but the seal member  60  can be removed gradually from the second end portion from the development device  3 . Therefore, the force necessary to remove the seal member  60  can be reduced. Accordingly, the force applied to the seal member  60  while being pulled is reduced, which can reduce the risk of damaging the sealing member  60  to such an extent that the seam member  60  cannot be removed while pulling the seal member  60 . 
     It is to be noted that the seal member  60  is not necessarily folded when welded. Sealing of the communicating area is not limited to thermal welding but can be any configuration as long as the seal member can reliably seal the communicating area and can be pulled out from the development device  3 . 
     It is to be noted that the configuration according to the fourteenth embodiment is applicable to relatively compact development devices using a three-pole magnet roller and a development sleeve of reduced diameter as in the first through seventh embodiments. However, the configuration of the fourteenth embodiments is also applicable to development devices in which the number of developer-carrying magnetic poles to generate a magnetic field to keep the developer on the development sleeve is more than four, for example, five as in the comparative example 2 shown in  FIG. 11 . In other words, the configuration of the fourteenth embodiment is also applicable to development devises including the supply path and the circulation path disposed beneath the supply path. More specifically, in the supply path, the developer is supplied onto the development sleeve while transported in the axial direction of the development sleeve. The circulation path receives the developer from the downstream end portion of the supply path in the developer transport direction, and the developer is collected from the development sleeve in the circulation path while transported in the axial direction of the development sleeve in the opposite direction to the developer transport direction in the supply path. Then, the developer reached the downstream end portion of the circulation path is sent to the upstream end portion of the supply path in the developer transport direction. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.