Abstract:
Described is a developing roller, which makes it possible to appropriately peel developer off the developing roller, even if the developing roller is a small-sized. The developing roller includes a non-magnetic developing sleeve and a magnetic member installed into an inner space of the developing sleeve and having plural magnetic poles including a peeling magnetic pole. When a reference position is defined as such a position that exhibits a maximum value of magnetic flux density to be generated in a normal direction by the peeling magnetic pole, a magnetic-flux density flat region within which the magnetic flux density is kept at substantially a constant value being closely approximate to the maximum value of the magnetic flux density and the reference position is included, exists while fulfilling an Equation indicated as follow:
 
 Z ≧7°
       where numeral Z indicates an angle of the developing roller in the magnetic-flux density flat region.

Description:
This application is based on Japanese Patent Application NO. 2008-131696 filed on May 20, 2008, with Japan Patent Office, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a developing device provided with a developing roller that bears developer including magnetic carrier and toner on its circumferential surface, so as to develop a latent image formed on an image bearing member, an image forming apparatus that is provided with the developing device, and the developing roller that bears the developer including magnetic carrier and toner on its circumferential surface, so as to develop the latent image formed on the image bearing member. 
     In an image forming process of the electro-photographic method, which employs two component developing agent (hereinafter, also referred to as developer) including magnetic carrier and toner, a latent image formed on a photoreceptor drum (image bearing member) is developed by applying toner included in the developer bone by the developing roller, so as to form a visible toner image on the photoreceptor drum concerned. 
     Generally speaking, the developing roller is constituted by a developing sleeve made of a non-magnetic material and a magnetic member provided with a plurality of fixed magnetic poles, to convey the developer to a developing magnetic pole disposed opposite to the photoreceptor drum so as to make the toner adhere onto the photoreceptor drum from the developing magnetic pole serving as one of the plurality of fixed magnetic poles. Further, the developer passed through the developing magnetic pole are peeled off the developing sleeve by the peeling action of a peeling magnetic pole, serving as another one of the plurality of fixed magnetic poles, and, after being sufficiently agitated, the developer are made to adhere onto the circumferential surface of the developing sleeve by an adhering action of a developer-layer forming magnetic pole, serving as still another one of the plurality of fixed magnetic poles. 
     Incidentally, the size minimization trend in the field of image forming apparatus that employs the electro-photographic process, including a copier, a printer, etc., has been considerably progressed in the recent years. Due to an influence of such the size minimization trend, the developing device that includes the developing roller has been minimized as smaller as possible. The problem to be arisen associating with the size minimization of the developing roller is a releasability of developer. The smaller the outer diameter of the developing roller is, the shorter the distance between the peeling magnetic pole and the developer-layer forming magnetic pole becomes. As a result, it becomes impossible to peel a sufficient amount of developer off the peeling magnetic pole, and accordingly, a residual amount of developer is attracted to the developing roller as it is. Therefore, it is impossible to supply the developer including a sufficient amount of toner to the developing magnetic pole, resulting in the density deterioration of the toner image to be formed on the photoreceptor drum. 
     To cope with the abovementioned problems, there has been set forth such a technology that adjusts the density of magnetic flux generated by the plurality of fixed magnetic poles provided in the magnetic member, so as to prevent the toner image from generating density deterioration. 
     For instance, according to the technology set forth in Tokkaihei 11-161029 (Japanese Non-Examined Patent Publication), the magnetic poles arranged on the circumferential surface of the developing roller from the conveyance magnetic pole (equivalent to the peeling magnetic pole) to the adhering magnetic pole (equivalent to the developer-layer forming magnetic pole) are set at the same polarity. Further, the magnetic flux density A at such a position that is rotated at 10° from the position of the adhering magnetic pole side  100 G of the conveyance magnetic pole, provided on the circumferential surface of the developing roller, in the reverse direction of the developing roller rotational direction is made to fulfill the Equation of (A−100)/10≦16, and the magnetic flux density B at such a position that is rotated at 10° from the position of the conveyance magnetic pole side  100 G of the adhering magnetic pole, provided on the circumferential surface of the developing roller, in the developing roller rotational direction is made to fulfill the Equation of (B−100)/10≦15. In other words, Tokkaihei 11-161029 sets forth such the technology that, considering the releasability of developer, the magnetic flux density around the peripheral space of the specific fixed magnetic pole is adjusted to a predetermined value. 
     According to the technology set forth in the Tokkaihei 11-161029, however, merely the magnetic flux density around the peripheral space of the conveyance magnetic pole (equivalent to the peeling magnetic pole) and/or around the peripheral space of the adhering magnetic pole (equivalent to the developer-layer forming magnetic pole) is adjusted, and the magnetic flux density at the conveyance magnetic pole becomes high. As a result, since the attractive force toward the center of the developing roller at the conveyance magnetic pole is getting large, the developer residing at that portion are attracted toward the developing roller, resulting in insufficient releasability of the developer. 
     SUMMARY OF THE INVENTION 
     To overcome the abovementioned drawbacks in conventional developing device, it is one of objects of the present invention to provide a developing roller and a developing device, which makes it possible to appropriately peel developer off the developing roller, even if the developing roller is a small-sized developing roller. 
     Accordingly, at least one of the objects of the present invention can be attained by any one of the developing devices, the image forming apparatuses and the developing rollers described as follows.
     (1) According to a developing device reflecting an aspect of the present invention, the developing device comprises: a developing roller to bear developer, including magnetic carriers and toner, on a circumferential surface thereof, so as to develop a latent image formed on an image bearing member; and a developer supplying section to supply the developer onto the developing roller; wherein the developing roller is provided with: a developing sleeve that is made of a non-magnetic material; and a magnetic member that is installed into an inner space of the developing sleeve and provided with a plurality of fixed magnetic poles including at least a peeling magnetic pole; and wherein, when a reference position is defined as such a position that exhibits a maximum value of a magnetic flux density to be generated in a normal direction by the peeling magnetic pole, a magnetic-flux density flat region within which the magnetic flux density is kept at substantially a constant value being closely approximate to the maximum value of the magnetic flux density and the reference position is included, exists while fulfilling an Equation indicated as follow:
 
 Z≧ 7°
   

     where numeral Z indicates an angle of the developing roller in the magnetic-flux density flat region.
     (2) According to another aspect of the present invention, in the developing device recited in item 1, the magnetic flux density within the magnetic-flux density flat region is equal to or greater than 98% of the maximum value.   (3) According to an image forming apparatus reflecting still another aspect of the present invention, the image forming apparatus comprises: an image bearing member to form a latent image on a circumferential surface thereof; a developing device that includes a developing roller to bear developer, including magnetic carriers and toner, on a circumferential surface thereof, so as to develop the latent image formed on the image bearing member; a transferring section to transfer a toner image formed on the image bearing member; and a fixing section to fix the toner image onto a sheet; wherein the developing roller is provided with: a developing sleeve that is made of a non-magnetic material; and a magnetic member that is installed into an inner space of the developing sleeve and provided with a plurality of fixed magnetic poles including at least a peeling magnetic pole; and wherein, when a reference position is defined as such a position that exhibits a maximum value of a magnetic flux density to be generated in a normal direction by the peeling magnetic pole, a magnetic-flux density flat region within which the magnetic flux density is kept at substantially a constant value being closely approximate to the maximum value of the magnetic flux density and the reference position is included, exists while fulfilling an Equation indicated as follow:
 
 Z≧ 7°
   

     where numeral Z indicates an angle of the developing roller in the magnetic-flux density flat region.
     (4) According to still another aspect of the present invention, in the image forming apparatus recited in item 3, the magnetic flux density within the magnetic-flux density flat region is equal to or greater than 98% of the maximum value.   (5) According to a developing roller reflecting still another aspect of the present invention, the developing roller that bears developer, including magnetic carriers and toner, on a circumferential surface thereof, so as to develop a latent image formed on an image bearing member, comprises: a developing sleeve that is made of a non-magnetic material; and a magnetic member that is installed into an inner space of the developing sleeve and provided with a plurality of fixed magnetic poles including at least a peeling magnetic pole; and wherein, when a reference position is defined as such a position that exhibits a maximum value of a magnetic flux density to be generated in a normal direction by the peeling magnetic pole, a magnetic-flux density flat region within which the magnetic flux density is kept at substantially a constant value being closely approximate to the maximum value of the magnetic flux density and the reference position is included, exists while fulfilling an Equation indicated as follow:
 
 Z≧ 7°
   

     where numeral Z indicates an angle of the developing roller in the magnetic-flux density flat region.
     (6) According to yet another aspect of the present invention, in the developing roller recited in item 5, the magnetic flux density within the magnetic-flux density flat region is equal to or greater than 98% of the maximum value.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
         FIG. 1  shows a center cross sectional schematic diagram indicating an internal configuration of an image forming apparatus embodied in the present invention; 
         FIG. 2  shows a cross sectional schematic diagram indicating a rough configuration of a developing device embodied in the present invention; 
         FIG. 3  shows an explanatory graph indicating a transition of a magnetic flux density on a developing roller from a peeling magnetic pole to a developer-layer forming magnetic pole, embodied in the present invention; 
         FIG. 4  shows an explanatory graph indicating a transition of a magnetic flux density in a peripheral area of a peeling magnetic pole, embodied in the present invention; 
         FIG. 5  shows an explanatory schematic diagram indicating an evaluation test pattern; and 
         FIG. 6  shows a graph indicating evaluation results of density deterioration of an image. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     &lt;Rough Configuration of Image Forming Apparatus&gt; 
       FIG. 1  shows a center cross sectional schematic diagram indicating an internal configuration of an image forming apparatus A. The image forming apparatus A, shown in  FIG. 1 , is called a tandem color image forming apparatus, and is constituted by a plurality of image forming sections  10 Y,  10 M,  10 C,  10 K, a belt-type intermediate transfer member  6 , a paper sheet feeding device  20  and a fixing device  30 . 
     On the upper section of the image forming apparatus A, an image reading device SC is mounted, so that an image residing on a document put on a platen cover (document plate) is read into a line image sensor by exposure-scanning actions performed by an optical system of an document image exposure-scanning section incorporated in the image reading device SC. Analogue signals acquired by photo-electric converting actions performed by the line image sensor are inputted into an image processing section so as to apply various kinds of image processing, such as an analogue processing, an analogue-to-digital conversion processing, a shading correction processing, an image compression processing, etc., to the analogue signals therein. Then, the processed digital image data are inputted into exposure sections  3 Y,  3 M,  3 C,  3 K. Incidentally, the line image sensor, the platen cover, the optical system of the document image exposure-scanning section and the image processing section are not clearly shown in the drawings. 
     The image forming section  10 Y, which forms an image of unicolor Y (Yellow), includes a charging electrode  2 Y, the exposure section  3 Y, a developing device  4 Y and a cleaning section  5 Y, which are respectively disposed in the peripheral space of an image bearing member  1 Y. Further, the image forming section  10 M, which forms an image of unicolor M (Magenta), includes a charging electrode  2 M, the exposure section  3 M, a developing device  4 M and a cleaning section  5 M, which are respectively disposed in the peripheral space of an image bearing member  1 M. Still further, the image forming section  10 C, which forms an image of unicolor C (Cyan), includes a charging electrode  2 C, the exposure section  3 C, a developing device  4 C and a cleaning section  5 C, which are respectively disposed in the peripheral space of an image bearing member  1 C. Yet further, the image forming section  10 K, which forms an image of unicolor K (Black), includes a charging electrode  2 K, the exposure section  3 K, a developing device  4 K and a cleaning section  5 K, which are respectively disposed in the peripheral space of an image bearing member  1 K. 
     Each of the developing devices  4 Y,  4 M,  4 C,  4 K accommodates two component developer including fine particle toner whose color is the corresponding one of unicolor Y (Yellow), unicolor M (Magenta), unicolor C (Cyan), and unicolor K (Black), and carriers. 
     The belt-type intermediate transfer member  6  is threaded on a plurality of rollers so as to rotatably support it. Further, the unicolor toner images Y, M, C, K formed on the image bearing members  1 Y,  1 M,  1 C,  1 K of the image forming sections  10 Y,  10 M,  10 C,  10 K are sequentially transferred onto the belt-type intermediate transfer member  6  one by one by the transferring actions performed by transferring sections  7 Y,  7 M,  7 C,  7 K, so as to form a full color toner image by superimposing the unicolor toner images Y, M, C, K. 
     One of paper sheets P accommodated in a paper sheet feeding cassette  21  of the paper sheet feeding device  20  is picked up by the paper sheet feeding section  22  and conveyed to a secondary transferring section  9  through pairs of paper sheet feeding rollers  23 ,  24 ,  25 ,  26 , and a pair of registration roller  27 , so that the full color toner image formed on the belt-type intermediate transfer member  6  is transferred onto corresponding one of paper sheets P. In this connection, configurations of paper sheet feeding cassettes  21 , which are arranged substantially in a vertical direction at the lower space of the image forming apparatus A so as to configure them as a three stage structure, are substantially the same. Further, configurations of paper sheet feeding sections  22 , which are respectively incorporated into the three stages of the paper sheet feeding cassettes  21 , are substantially the same, as well. Hereinafter, the paper sheet feeding cassette  21  including the paper sheet feeding section  22  is called the paper sheet feeding device  20 . Successively, the paper sheet P, on which the full color toner image is transferred, is further conveyed into the fixing device  30  (fixing section), in which the paper sheet P is tightly clipped by a pair of fixing rollers so as to apply heat and pressure onto both the paper sheet P and the full color toner image, to fix the full color toner image onto the paper sheet P. Still successively, the paper sheet P, on which the full color toner image is fixed, is tightly clipped by a pair of ejecting rollers  28  so as to ejects it onto an ejecting tray  29  disposed outside the apparatus. 
     On the other hand, after the full color toner image has been transferred onto the paper sheet P from the belt-type intermediate transfer member  6  and the paper sheet P has been separated from the belt-type intermediate transfer member  6  by employing the curvature separating action, the residual toner remaining on the belt-type intermediate transfer member  6  are removed by the cleaning action performed by a belt cleaning section  8 . 
     In this connection, although the color image forming operation has been exemplified for describing the image forming apparatus A in the foregoing, it is needless to say that the monochrome image forming operation is also included within the scope of the present invention. Hereinafter, each of the developing devices  4 Y,  4 M,  4 C,  4 K is also referred to as a developing device  4  as its general term, and further, each of the image bearing member  1 Y,  4 M,  4 C,  4 K is also referred to as a image bearing member  1  as well as the above. Further, hereinafter for simplicity, the above terminological rule will be also applied to the other terms included in the four color image forming sections. 
     &lt;Rough Configuration of Developing Device&gt; 
       FIG. 2  shows a cross sectional schematic diagram indicating a rough configuration of the developing device  4  embodied in the present invention. A housing body of the developing device  4  is constituted by a lower casing  40 , covering the lower space, and an upper casing  50 , covering the upper space, in such a manner that the upper casing  50  can be opened and closed relative to the lower casing  40 . Further, the developing device  4  is constituted by a developing roller  41 , a guide member  46 , a toner supplying roller  45 , a first agitating member  43 , a second agitating member  44 , etc. Still further, the guide member  46  is constituted by a guiding plate  461  and a pedestal section  462 . 
     Each of the developing devices  4 Y,  4 M,  4 C,  4 K accommodates the two component developing agent including the fine particle toner whose color is the corresponding one of unicolor Y (Yellow), unicolor M (Magenta), unicolor C (Cyan), and unicolor K (Black), which are unicolors being different from each other, and the carriers. Concretely speaking, the two component developing agent includes the carriers, each particle of which is a ferrite core coated with an insulating resin material, and the toner that includes polyester as its main gradients and various kinds of additives including a coloring agent such as a pigment, a carbon black, etc., a charge controlling agent, a silica, a titanium oxide, etc. 
     The particle diameter and the saturation magnetization of the carriers are set at a value in a range of 10-50 μm and another value in a range of 10-80 emu/g, respectively. On the other hand, the particle diameter of the toner is set at a value in a range of 4-10 μm, while the charging characteristic of the toner is a negative charging characteristic and the average charge amount is set at a value in a range of −20-−60 μC/g. Further, the present embodiment employs such the two component developing agent that includes the above-specified carriers and toner, which are mixed with each other so as to set the toner density at a value in a range of 4-10%/mass (percentage by mass). 
     As shown in  FIG. 2 , the developing roller  41 , the first agitating member  43 , the second agitating member  44 , the toner supplying roller  45 , the guide member  46 , etc., are arranged in the lower space of the developing device  4 , enclosed by the lower casing  40 . Further, the toner supplying roller  45 , the first agitating member  43  and the second agitating member  44  are driven by an agitating motor (not shown in the drawings), serving as a common driving source of them. The first agitating member  43  and the second agitating member  44  are rotated at a rotational velocity in a range of 250-700 rpm. 
     The lower casing  40  creates a developer supplying chamber  401  that accommodates the first agitating member  43  and a developer agitating chamber  402  that accommodates the second agitating member  44 . Concretely speaking, a partition wall  403  protruded from the bottom section of the lower casing  40  forms both the developer supplying chamber  401  and the developer agitating chamber  402 , which are separated from each other while putting the partition wall  403  between them. 
     Further, a toner density sensor  406  is disposed at such a position that is located downstream the conveying direction of the developer and just below the second agitating member  44  on the bottom surface of the lower casing  40 . The toner density sensor  406 , complying with the magnetic permeability detecting method, detects a property based on the magnetic permeability of the developer (carriers) serving as a material under test, and outputs the detected property as, for instance, a voltage value. Compared to the reference toner density, the higher the toner density detected by the toner density sensor  406  is, the lower the output value becomes, while, the lower the toner density detected by the toner density sensor  406  is, the higher the output value becomes. As abovementioned, it is possible to find the toner density from the output value of the toner density sensor that complies with the magnetic permeability detecting method. Then, corresponding to the toner density detected, a suitable amount of toner is supplied from a toner supplying section (not shown in the drawings) so as to keep the toner density at a predetermined value. 
     Still further, the developing roller  41  is constituted by a developing sleeve  41 A that is rotatable, a magnet roller (magnet member)  41 B that is fixed and generates magnetic fields. 
     At the opposing near point between the developing sleeve  41 A and the magnet roller  41 B, the developing sleeve  41 A rotates upward from the bottom space to the upper space, while, the first agitating member  43  rotates downward from the upper space to the bottom space. 
     Still further, a developer amount regulating member  51  is fixed at the ceiling section located inside the upper casing  50 . Accordingly, a wide space, serving as a developer flow rectifying space W, is created between the guiding plate  461  and the inner wall surface of the upper casing  50 , so as to make it possible to accommodate a large amount of developer therein. 
     The developing roller  41  is disposed at such a position that opposes to the image bearing member  1  and is rotated by a driving motor (not shown in the drawings) through a coupling gear (not shown in the drawings). Further, a developing bias voltage E, created by superimposing an alternate voltage Vac supplied from an alternate electric power source E 1  and a direct-current voltage Vdc supplied from a direct-current electric power source E 2  with each other, is applied to the developing sleeve  41 A. For instance, as the practical operating conditions, the rotating velocity of the developing roller  41  is set at a value in a range of 200-1000 mm/sec as a line velocity of its circumferential surface, and the developing bias voltage E is set at such a value that is specified by the alternate voltage Vac in a range of 0.5-2.0 kvp-p, a frequency of the alternate voltage Vac in a range of 2-7 kHz, and the direct-current voltage Vdc in a range of −200-−700 Volts, as its output voltage. 
     The magnet roller  41 B is disposed inside the developing sleeve  41 A, and provided with seven magnetic poles N 1 , N 2 , N 3 , N 4 , S 1 , S 2 , S 3 . Concretely speaking, the magnetic pole N 1  serves as a developing magnetic pole, the magnetic pole N 2  serves as a peeling magnetic pole and the magnetic pole N 3  serves as a developer-layer forming magnetic pole. Further, the developer amount regulating member  51  is disposed at a position in the vicinity of the magnetic pole N 3  of the magnet roller  41 B. 
     In the present embodiment, the guide member  46  is in charge of performing a lift pumping function for supplying the developer to the developing sleeve  41 A without providing a conventional lift pumping magnetic pole. Accordingly, since the lift pumping magnetic pole is excluded from the developing device  4 , it becomes possible to reduce the rotational torque to be incurred to the developing sleeve  41 A. In other words, it becomes possible to reduce the mechanical stresses to be generated associated with the operations for conveying and agitating the developer around the developing roller  41 , and as a result, it becomes possible to improve the durability of the developer and/or the aging stability of the image quality. 
     Further, the magnetic poles N 2 , N 3 , which are disposed at positions being adjacent to each other among the plural magnetic poles, have the same polarity so as to form a repulsive magnetic field. The magnetic pole N 2  serves as the peeling magnetic pole for peeling the developer off the developing sleeve  41 A. Further, the developer supplied by the first agitating member  43  is conveyed by sliding the developer on the upper surface of the guiding plate  461 , so that the magnetic pole N 3 , serving as the developer-layer forming magnetic pole, attracts the developer, so as to adhere the developer onto the circumferential surface of the developing sleeve  41 A. 
     Still further, the first agitating member  43  agitates and conveys the developer conveyed from the second agitating member  44 , so as to uniformly supply the developer onto the developing roller  41 . Each of the first agitating member  43  and the second agitating member  44  is a screw member formed in a spiral shape. The first agitating member  43  and the second agitating member  44  are arranged in parallel to each other, and, mix and agitate flesh developer, supplied from the toner supplying section (not shown in the drawings), with the developer refluxed (recycled) from the developing sleeve  41 A, so as to convey the mixed developer to the upstream space of the first agitating member  43 . 
     The first agitating member  43  conveys the developer in an axial direction of its rotation, and at the same time, emits the developer in a direction orthogonal to the rotational axis so as to agitate the developer. 
     Yet further, the guide member  46  that not only separates the developer, which are peeled off the developing sleeve  41 A and conveyed through the lower space in the direction indicated by the outlined arrow shown in  FIG. 2 , from the other developer, which are about to supply onto the developing sleeve  41 A and conveyed through the upper space, but also accumulates the developer currently supplied from the toner supplying roller  45 , thereon, to guide the accumulated developer towards the developing roller  41 , is disposed in the vicinity of the opposing near position between the developing sleeve  41 A and the first agitating member  43 . 
     &lt;Releasability of Developer&gt; 
     Next, based on the magnetic flux density of the magnetic field to be generated by each of the peeling magnetic pole N 2  and the developer-layer forming magnetic pole N 3 , the releasability of the developer residing on the developing roller  41  will be detailed in the following.  FIG. 3  shows an explanatory graph indicating a transition of the magnetic flux density on the developing roller  41  from the peeling magnetic pole N 2  to the developer-layer forming magnetic pole N 3 . In the graph shown in  FIG. 3 , the horizontal axis represents an angle of current position on the developing roller  41  from the peeling magnetic pole N 2  (in  FIG. 3 , the right direction of the horizontal axis coincides with the rotational direction of the developing sleeve  41 A), while the vertical axis represents a magnetic flux density in a normal direction of the developing roller  41 . 
     Since a value of the magnetic flux density is kept low at an angle in an intermediate range between the peeling magnetic pole N 2  and the developer-layer forming magnetic pole N 3  as shown in  FIG. 3 , a centripetal force exerted towards the center of the developing roller  41  (hereinafter, referred to as a centripetal force Fr) is small. Accordingly, the developer residing on the intermediate area between the peeling magnetic pole N 2  and the developer-layer forming magnetic pole N 3  can be peeled off the developing sleeve  41 A. However, if a tangential force exerted towards the peeling magnetic pole N 2  in the circumferential direction within the intermediate region between the peeling magnetic pole N 2  and the developer-layer forming magnetic pole N 3  (hereinafter, referred to as a tangential force Fθ) is small, the developer, peeled off the developing roller  41 , are attracted and attached by/to the developer-layer forming magnetic pole N 3  from the residential space. As a result, the density deterioration of the toner image would occur, due to the inability for supplying developer including sufficient amount of toner to the developer-layer forming magnetic pole N 3  and/or the developing magnetic pole N 1 . Specifically, with respect to such the developing roller  41  that is provided with the developing sleeve  41 A whose outer diameter is set at a value being equal to or smaller than 30 mm (also equivalent to the outer diameter of the developing roller  41 ), since the distance between the peeling magnetic pole N 2  and the developer-layer forming magnetic pole N 3  is relatively short, the developer passed through the peeling magnetic pole N 2  is liable to adhere the developer-layer forming magnetic pole N 3 . 
     To overcome the abovementioned drawback, in the present embodiment, the magnetic flux density at the peeling magnetic pole N 2  is made to be high as shown in  FIG. 3 , and the magnetic flux density in a region Ra of the peeling magnetic pole N 2  is made to be high, to direct the tangential force Fθ towards the peeling magnetic pole N 2  so as to attract the developer to the peeling magnetic pole N 2 . However, when the magnetic flux density in a region Rb, which includes the maximum value of the magnetic flux density to be generated by the peeling magnetic pole N 2 , is kept at the high level as shown in  FIG. 3 , the centripetal force Fr generated by the peeling magnetic pole N 2  is getting strong, and as a result, it becomes difficult to peel the developer off the developing roller  41  (the developer hardly release from the circumferential surface of the developing sleeve  41 A). 
       FIG. 4  shows an explanatory graph indicating a transition of the magnetic flux density in the peripheral area of the peeling magnetic pole N 2 . When the maximum value of the magnetic flux density to be generated by the peeling magnetic pole N 2  is relatively high as indicated by a solid line K 1  shown in  FIG. 4 , the releasability of the developer is getting worse. Accordingly, by lowering a maximum value X of the magnetic flux density to be generated by the peeling magnetic pole N 2 , a flat region T in which the magnetic flux density is substantially the same as the maximum value X (the flat region T includes a position X 1 , serving as a reference position at which the magnetic flux density is maximum) is created, so as to weaken the centripetal force Fr. It is preferable that, by making the magnetic flux density abruptly reduce in the region from the peeling magnetic pole N 2  to the developer-layer forming magnetic pole N 3  (region Ra shown in  FIG. 4 ), the region in which the magnetic flux density is low is formed between the peeling magnetic pole N 2  and the developer-layer forming magnetic pole N 3 , so as to make the tangential force Fθ directed towards the peeling magnetic pole N 2  strong (greater). 
     The present inventor has verified the width of the flat region T that causes no problem of the density deterioration of the image and is shown in  FIG. 4 , through the experiments. In this connection, hereinafter, the flat region T is defined as such a region that fulfills the following Equation.
 
 X≧Y≧ 0.98×( X )
 
     As the conditions for the experiments, with respect to the outer diameter of the developing roller  41 , the alternate voltage Vac to be generated by the alternate electric power source E 1 , the direct-current voltage Vdc to be generated by the direct-current electric power source E 2  and a charged voltage Vo of the image bearing member, the three combinations of the those values are established as shown in Table 1 indicated as follow. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Experi. 1 
                 Experi. 1 
                 Experi. 1 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Outer diameter of 
                 25 
                 25 
                 30 
               
               
                   
                 developing roller [mm] 
               
               
                   
                 Vac [kV] 
                 1.0 
                 0.9 
                 1.0 
               
               
                   
                 Vdc [V] 
                 −450 
                 −510 
                 −450 
               
               
                   
                 Vo [V] 
                 −600 
                 −660 
                 −600 
               
               
                   
                   
               
               
                   
                 Experi.: Experiment 
               
             
          
         
       
     
     Further, changing an angle Z of the developing roller  41  in the flat region T (value of angle of the flat region T shown in  FIG. 4 ), in regard to the 10 concerned angles of 3.0°, 4.5°, 6.5°, 7.0°, 10.0°, 11.5°, 13.0°, 14.0°, 15.0° and 17.0°. In this connection, the changes of the angle Z were achieved by changing the shape and/or the material of the peeling magnetic pole N 2 . Still further, the evaluations of the density deterioration of the image were implemented by forming an evaluation pattern shown in  FIG. 5  onto the image bearing member  1  (the solid line arrow shown in  FIG. 5  indicates the rotational direction of image bearing member  1 , while the hatched area indicates the toner adhered area), based on a value (ghost value) derived by subtracting the density value of an area TD 2  from that of an area TD 1  in the evaluation pattern. Density values were those detected by the toner density sensor  406 . As shown in  FIG. 5 , none of toner adheres to an area located in the upstream side of the rotational direction of the image bearing member  1  relative to the area TD 1 , while toner adheres to another area located in the upstream side of the rotational direction of the image bearing member  1  relative to the area TD 2 . 
       FIG. 6  shows a graph indicating the evaluation results of the density deterioration of the image. In  FIG. 6 , the horizontal axis represents the angle Z of the developing roller  41  corresponding to the flat region T, while the vertical axis represents the ghost value. Although the present inventor has recognized that the area in which the ghost value is equal to or smaller than 0.03 exhibits good releasability of the developer and causes no density deterioration of the image, the present inventor has also found that the area in which the angle Z is equal to or greater than 7° exhibits specifically good results in the experiments  1  through the experiments  3 , irrespective of the outer diameter of the developing roller  41 , and irrespective of the electric field conditions. 
     As described in the foregoing referring to the  FIG. 3  through  FIG. 6 , the flat region T, in which the magnetic flux density is kept substantially the same as the maximum value X, is made to exist within a certain predetermined region, so as to weaken the centripetal force Fr to be generated by the peeling magnetic pole N 2 . Further, by making the magnetic flux density abruptly reduce in a suitable region towards the developer-layer forming magnetic pole N 3  from the peeling magnetic pole N 2 , a specific region, in which the magnetic flux density is low, is created between the peeling magnetic pole N 2  and the developer-layer forming magnetic pole N 3 . According to the abovementioned, the magnetic field generated by the peeling magnetic pole N 2  does not overlap with that generated by the developer-layer forming magnetic pole N 3 , and as a result, it becomes possible to appropriately peel the developer off the developing roller  41 , even if the developing roller  41  is a small-sized developing roller. 
     Incidentally, although, referring to the drawings, the preferred embodiment has been described in the foregoing, the scope of the present invention is not limited to the above-described embodiment. Modifications and additions made by a skilled person without departing from the spirit and scope of the invention shall be included in the scope of the present invention. 
     According to a developing device, an image forming apparatus and a developing roller, embodied in the present invention, it becomes possible to appropriately peel developer off the developing roller, even if the developing roller is a small-sized developing roller. 
     While the preferred embodiments of the present invention have been described using specific term, such description is for illustrative purpose only, and it is to be understood that changes and variations may be made without departing from the spirit and scope of the appended claims.