Patent Publication Number: US-10331061-B2

Title: Developing device

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a developing device including a developer carrying member rotatable while carrying a developer, and relates to an image forming apparatus, including the developing device, such as a copying machine, a printer, a facsimile machine or a multi-function machine having a plurality of functions of these machines. 
     An image forming apparatus of an electrophotographic type or an electrostatic recording type includes a developing device for developing an electrostatic latent image, with a developer such as toner, formed on a photosensitive drum as an image bearing member. The developing device includes a developing sleeve as a developer carrying member rotatable while carrying a developer and supplies, to the photosensitive drum, the developer carried on the developing sleeve. 
     In the case of such a developing device, there is a liability that air flows into a developing container constituting the developing device due to rotation of the developing sleeve and atmospheric pressure in the developing container increases and thus the developer in the developing container is scattered to an outside of the developing container. For this reason, a constitution in which an inner cover is provided between an outer cover of the developing container and the developing sleeve and the air flowing from between the developing sleeve and the inner cover into the developing container is discharged from between the inner cover and the outer cover has been proposed (Japanese Laid-Open Patent Application (JP-A) 2015-72331). 
     However, in the case of the constitution disclosed in JP-A 2015-72331, there is a liability that the air containing the developer is discharged to an outside of the developing container from an inflow path, between the developing sleeve and the inner cover, for permitting flowing of the air into the developing container. For this reason, there is a possibility that scattering of the developer cannot be sufficiently suppressed. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide a constitution capable of sufficiently suppressing scattering of a developer. Specifically, an object of the present invention is to provide a developing device capable of suppressing the scattering of the developer from the developing device. 
     According to an aspect of the present invention, there is provided a developing device comprising: an accommodating casing configured to accommodate a developer; a rotatable developer carrying member provided in the accommodating casing and configured to develop, in a developing region, an electrostatic latent image formed on an image bearing member; a regulating portion provided below the developer carrying member with respect to a vertical direction and configured to regulate an amount of the developer on the developer carrying member; a magnetic flux generating portion provided inside the developer carrying member and including a first magnetic pole provided downstream of the developing region with respect to a rotational direction of the developer carrying member and a second magnetic pole which is provided adjacently downstream of the first magnetic pole with respect to the rotational direction and which has a polarity identical to a polarity of the first magnetic pole; and a cover portion provided downstream of the developing region and upstream of a maximum magnetic flux density position of the second magnetic pole with respect to the rotational direction the cover portion being disposed between the casing and the developer carrying member over a rotational axis direction of the developer carrying member with a gap between itself and the casing and with a gap between itself and the developer carrying member, wherein a downstream end of the cover portion with respect to the rotational direction is in a side upstream, with respect to the rotational direction, of a minimum magnetic flux density position between the first magnetic pole and the second magnetic pole with respect to the rotational direction. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an image forming apparatus according to a First Embodiment. 
         FIG. 2  is a schematic sectional view of an image forming portion in the First Embodiment. 
         FIG. 3  is a schematic cross-sectional view of a developing device in the First Embodiment. 
         FIG. 4  is a schematic longitudinal sectional view of the developing device in the First Embodiment. 
         FIG. 5  is a schematic sectional view of a supplying device and the developing device in the First Embodiment. 
         FIG. 6  is a sectional view schematically showing an air flow of a developing device in a comparison example. 
         FIG. 7  is a sectional view of a periphery of a developing sleeve of the developing device in the First Embodiment. 
         FIG. 8  is a sectional view schematically showing an air flow at a periphery of the developing sleeve of the developing device in the First Embodiment. 
         FIG. 9  is a sectional view schematically showing an air flow at a periphery of a merging path of the developing device in the First Embodiment. 
         FIG. 10  is a graph showing a result of a comparative experiment. 
         FIG. 11  is a sectional view of a periphery of a developing sleeve of a developing device according to a Second Embodiment. 
         FIG. 12  is a sectional view schematically showing an air flow at the periphery of the developing sleeve of the developing device in the Second Embodiment. 
         FIG. 13  is a sectional view of a periphery of a developing sleeve of a developing device according to a Third Embodiment. 
         FIG. 14  is a sectional view schematically showing an air flow at the periphery of the developing sleeve of the developing device in the Third Embodiment. 
         FIG. 15  is a schematic sectional view of a developing device according to a Fourth Embodiment. 
         FIG. 16  is a schematic sectional view of a developing device according to a Fifth Embodiment. 
         FIG. 17  is a longitudinal sectional view of a periphery of a developing sleeve of a developing device according to a Sixth Embodiment. 
         FIG. 18  is a cross-sectional view of the periphery of the developing sleeve in the Sixth Embodiment. 
         FIG. 19  is a cross-sectional view of a periphery of a developing sleeve according to a Seventh Embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;First Embodiment&gt; 
     The First Embodiment (Embodiment 1) will be described with reference to  FIGS. 1 to 10 . First, a general structure of an image forming apparatus in this embodiment will be described using  FIGS. 1 and 2 . 
     [Image Forming Apparatus] 
     An image forming apparatus  100  in this embodiment is a tandem (-type) full-color printer of an electrophotographic type, in which four image forming portions PY, PM, PC and PK each including a photosensitive drum  1  as an image bearing member are provided. The image forming apparatus  100  forms a toner image (image) on a recording material depending on an image signal from a host device such as an original reading device (not shown) connected with an apparatus main assembly  100 A or a personal computer communicatably connected with the apparatus main assembly  100 A. As the recording material, a sheet material such as a sheet, a plastic film or a cloth can be cited. Further, the image forming portions PY, PM, PC and PK form toner images of yellow, magenta, cyan and black, respectively. 
     The four image forming portions PY, PM, PC and PK provided in the image forming apparatus  100  have the substantially same constitution except that colors of developers are different from each other. Accordingly, the image forming portion PY will be described as representative and other image forming portions will be omitted from description. 
     As shown in  FIG. 2 , at the image forming portion PY, a cylindrical photosensitive member as the image bearing member, i.e., the photosensitive drum  1 , is provided. The photosensitive drum  1  is rotationally driven in an arrow direction in the figure. At a periphery of the photosensitive drum  1 , a charging roller  2  as a charging means, a developing device  4 , a primary transfer roller  52  as a transfer means, and a cleaning device as a cleaning means are provided. Below the photosensitive drum  1  in the figure, an exposure device (a laser scanner in this embodiment) 3 as an exposure means is provided. 
     Above the respective image forming portions in  FIG. 1 , a transfer device  5  is provided. In the transfer device  5 , an endless intermediary transfer belt  51  as an intermediary transfer member is stretched by a plurality of rollers and is constituted so as to be circulated (rotated) in an arrow direction. The intermediary transfer belt  51  carries and feeds the toner images which are primary-transferred on the intermediary transfer belt  51  as described later. At a position opposing an inner secondary transfer roller  53 , of the rollers stretching the intermediary transfer belt  51 , while sandwiching the intermediary transfer belt  51  between itself and the inner secondary transfer roller  53 , an outer secondary transfer roller  54  as a secondary transfer means is provided and constitutes a secondary transfer portion T 2  for transferring the toner images from the intermediary transfer belt  51  onto the recording material. A fixing device  6  is provided downstream of the secondary transfer portion T 2  with respect to a recording material feeding direction. 
     At a lower portion of the image forming apparatus  100  is a cassette  9  in which the recording material S is accommodated. The recording material S fed from the cassette  9  is fed toward a registration roller pair  92  by a feeding roller pair  91 . A leading end of the recording material S abuts against the registration roller pair  92  which is in a rest state, and forms a loop, so that oblique movement of the recording material S is corrected. Thereafter, rotation of the registration roller pair  92  is started in synchronism with the toner images on the intermediary transfer belt  51 , so that the recording material S is fed to the secondary transfer portion T 2 . 
     A process of forming, for example, a four-color-based full-color image by the image forming apparatus  100  constituted as described above will be described. First, when an image forming operation is started, a surface of a rotating photosensitive drum  1  is electrically charged uniformly by the charging roller  2 . Then, the photosensitive drum  1  is exposed to laser light, corresponding to an image signal, emitted from the exposure device  3 . As a result, an electrostatic latent image corresponding to the image signal is formed on the photosensitive drum  1 . The electrostatic latent image on the photosensitive drum  1  is visualized by toner as the developer accommodated in the developing device  4  and is formed in a visible image (toner image). 
     The toner image formed on the photosensitive drum  1  is primary-transferred onto the intermediary transfer belt  51  at a primary transfer portion T 1  ( FIG. 2 ) constituted between the photosensitive drum  1  and a primary transfer roller  52  provided while sandwiching the intermediary transfer belt  51  between itself and the photosensitive drum  1 . At this time, to the primary transfer roller  52 , a primary transfer bias is applied. Toner (transfer residual toner remaining on the surface of the photosensitive drum  1  after the primary transfer is removed by the cleaning device  7 . 
     Such an operation is successively performed at the respective image forming portions for yellow, magenta, cyan and black, so that the four color toner images are superposed on the intermediary transfer belt  51 . Thereafter, in synchronism with timing of toner image formation, the recording material S accommodated in the cassette  9  is fed to the secondary transfer portion T 2 . Then, by applying a secondary transfer bias to the outer secondary transfer roller  54 , the four color toner images are secondary-transferred altogether from the intermediary transfer belt  51  onto the recording material S. Toner remaining on the intermediary transfer belt  51  without being completely transferred onto the recording material S at the secondary transfer portion T 2  is removed by an intermediary transfer belt cleaner  55 . 
     Then, the recording material S is fed to the fixing device  6  as a fixing means. In the fixing device  6 , a fixing roller  61  including a heat source such as a halogen heater and a pressing roller  62  are provided, and a fixing nip is formed by the fixing roller  61  and the pressing roller  62 . The recording material S on which the toner recording materials are transferred is passed through the fixing nip, so that the recording material S is heated and pressed. Then, the toners on the recording material S are melted and mixed with each other and are fixed as a full-color image on the recording material S. Thereafter, the recording material S is discharged onto a discharging tray  102  by a discharging roller  101 . As a result, a series of image forming process operations is ended. 
     Incidentally, the image forming apparatus  100  in this embodiment is also capable of forming a single-color image, such as a black (monochromatic) image, or a multi-color image by using the image forming portion(s) for a desired single color or for some of the four colors. 
     [Developing Device] 
     A detailed structure of the developing device  4  will be described using  FIGS. 3 and 4 . The developing device  4  includes a developing container  41  for accommodating non-magnetic toner and a magnetic carrier and includes a developing sleeve  44  as a developer carrying member rotating while carrying the developer in the developing container  41 . In the developing container  41 , feeding screws  43   a  and  43   b  as developer feeding members for circulating the developer in the developing container  41  while stirring and feeding the developer in the developing container  41  are provided. Inside the developing sleeve  44 , a magnet  44   a  as a maximum flux generating means including a plurality of magnetic poles with respect to a circumferential direction is non-rotatably provided. Further, a developing blade  42  as a regulating member for forming a thin layer of the developer on a surface of the developing sleeve  44  is provided. 
     Inside the developing container  41 , a substantially central portion thereof is partitioned into left and right portions with respect to a horizontal direction, i.e., into a stirring chamber  41   b  and a developing chamber  41   a  by a partition wall  41   c  extending in a direction perpendicular to the surface of the drawing sheet of  FIG. 3 , and the developer is accommodated in the developing chamber  41   a  and the stirring chamber  41   b . In the developing chamber  41   a  and the stirring chamber  41   b , the feeding screws  43   a  and  43   b  are disposed, respectively. At end portions of the partition wall  41   c  with respect to a longitudinal direction (i.e., at end portions of the developing sleeve  44  with respect to a rotational axis direction, left side and right side of  FIG. 4 ), delivering portions  41   d  and  41   e  for permitting passing of the developer between the developing chamber  41   a  and the stirring chamber  41   b  are provided. 
     Each of the feeding screws  43   a  and  43   b  is formed by providing a helical blade as a feeding portion around a shaft (rotation shaft) of a magnetic material. Further, the feeding screw  43   b  is provided, in addition to the helical blade, with stirring ribs  43   b   1  each having a predetermined width with respect to a developer feeding direction so as to project from the shaft in a radial direction of the shaft. The stirring ribs  43   b   1  stir the developer with rotation of the shaft. 
     The feeding screw  43   a  is disposed at a bottom portion of the developing chamber  41   a  along the rotational axis direction of the developing sleeve  44 , and feeds the developer to the developing sleeve  44  while feeding the developer in the developing chamber  41   a  along an axial direction by rotating the rotation shaft by an unshown motor. The developer which is carried on the developing sleeve  44  and of which toner is consumed in a developing step is collected in the developing chamber  41   a.    
     The feeding screw  43   b  is disposed at a bottom portion of the stirring chamber  41   b  along the rotational axis direction of the developing sleeve  44 , and feeds the developer in the stirring chamber  41   b  along an axial direction in a direction opposite to the developer feeding direction of the feeding screw  43   a . The developer is fed by the feeding screws  43   a  and  43   b  in this manner, and is circulated in the developing container  41  through the delivering portions  41   d  and  41   e.    
     At an upstream end portion of the stirring chamber  41   b  with respect to the developer feeding direction of the feeding screw  43   b , a developer supply opening  46  permits supply of the developer containing the toner into the developing container  41 . The developer supply opening  46  is connected with a supplying and feeding portion  83  of a developer supplying device  80  shown in  FIG. 5  and described later. Accordingly, a developer for supply is supplied from the developer supplying device  80  into the stirring chamber  41   b  through the supplying and feeding portion  83  and the developer supply opening  46 . The feeding screw  43   b  feeds the developer supplied through the developer supply opening  46  and the developer which has already been in the stirring chamber  41   b  while stirring these developers, so that a toner content (concentration) is uniformized. 
     Accordingly, by feeding forces of the feeding screws  43   a  and  43   b , the developer in the developing chamber  41   a  in which the toner is consumed in the developing step and thus the toner content is lowered and moved into the stirring chamber  41   b  through one delivering portion  41   d  (left side of  FIG. 4 ). Then, the developer moved in the stirring chamber  41   b  is fed while being stirred with the supplied developer and is moved into the developing chamber  41   a  though the other delivering portion  41   e  (right side of  FIG. 4 ). 
     The developing chamber  41   a  of the developing container  41  is provided with an opening  41   h  at a position corresponding to an opposing region (developing region) A opposing the photosensitive drum  1 , and in this opening  41   h , the developing sleeve  44  is rotatably provided so as to be partially exposed in a direction of the photosensitive drum  1 . On the other hand, the magnet  44   a  incorporated in the developing sleeve  44  is non-rotationally fixed. Such a developing sleeve  44  is rotated by an unshown motor, and is capable of feeding the developer to the opposing region A, and feeds the developer to the photosensitive drum  1  in the opposing region A. In this embodiment, the developing sleeve  44  is formed, in a cylindrical shape, of a non-maximum material such as aluminum or stainless steel. The developing sleeve  44  rotates from below toward above with respect to a direction of gravitation in the opposing region A, i.e., rotates in a counterclockwise direction of  FIG. 3 . 
     In a side upstream of the opening  41   h  with respect to the rotation direction of the developing sleeve  44 , the developing blade  42  as a regulating member for regulating an amount of the developer carried on the developing sleeve  44  is fixed. In this embodiment, the developing sleeve  44  rotates in the opposing region A from below toward above with respect to the direction of gravitation, and therefore, the developing blade  42  is positioned below the opposing region A with respect to the direction of gravitation. 
     The magnet  44   a  includes, as shown in  FIG. 3 , 5 magnetic poles in total consisting of a plurality of magnetic poles S 1 , S 2 , S 3 , N 1  and N 2  with respect to a circumferential direction and is formed in a roller shape. The developer in the developing chamber  41   a  is supplied to the developing sleeve  44  by the feeding screw  43   a , and the developer supplied to the developing sleeve  44  is carried in a predetermined amount on the developing sleeve  44  by a magnetic field generated by an attracting magnetic pole S 2  of the magnet  44   a , and forms a developer accumulating portion. 
     The developer on the developing sleeve  44  passes through the developer accumulating portion by rotation of the developing sleeve  44  and is erected by a regulating magnetic pole N 1 , and a layer thickness thereof is regulated by the developing blade  42  opposing the regulating magnetic pole N 1 . Then, the developer subjected to the layer thickness regulation is fed to the opposing region A opposing the photosensitive drum  1  and is erected by a developing magnetic pole S 1 , and forms a magnetic chain. This magnetic chain contacts the photosensitive drum  1  rotating in the same direction as the rotational direction of the developing sleeve  44  in the opposing region A, so that the electrostatic latent image is developed into the toner image with the charged toner. 
     Thereafter, the developer on the developing sleeve  44  is fed into the developing container  41  by the rotation of the developing sleeve  44  while attraction of the developer to the surface of the developing sleeve  44  is maintained by a feeding magnetic pole N 2 . Then, the developer carried on the developing sleeve  44  is peeled off the surface of the developing sleeve  44  by a peeling magnetic pole S 3  and is collected in the developing chamber  41   a  of the developing container  41 . 
     In the developing container  41 , as shown in  FIG. 4 , an inductance sensor  45  as a toner content sensor for detecting a toner content in the developing container  41  is provided. In this embodiment, the inductance sensor  45  is provided downstream of the stirring chamber  41   b  with respect to the developer feeding direction. 
     [Developing Supplying Device] 
     The developer supplying device  80  will be described using  FIG. 5 . The developer supplying device  80  includes an accommodating container  8  for accommodating the developer for supply and includes a supplying mechanism  81  and a supplying and feeding portion  83 . The accommodating container  8  has a constitution such that a helical groove is provided on an inner wall of a cylindrical container, so that a feeding force for feeding the developer in a longitudinal direction (rotational axis direction) by rotation of the accommodating container  8  itself. The accommodating container  8  is connected with the supplying mechanism  81  at a downstream end portion thereof with respect to the developer feeding direction. The supplying mechanism  81  includes a pump portion  81   a  for discharging the developer, fed from the accommodating container  8 , through a discharge opening  82 . The pump portion  81   a  is formed in a bellow shape and changes in volume by being rotationally driven, so that air pressure generates and thus the developer fed from the accommodating container  8  is discharged through the discharge opening  82 . 
     To the discharge opening  82 , an upstream end portion of the supplying and feeding portion  83  is connected, and a lower end portion of the supplying and feeding portion  83  is connected to a developer supply opening  46  of the developing device  4 . That is, the developer supplying and feeding portion  83  communicates the discharge opening  82  and the developer supply opening  46  with each other. Accordingly, the developer discharged through the discharge opening  82  by the pump portion  81   a  passes through the developer supplying and feeding portion  83  and is supplied into the developing container  41  of the developing device  4 . 
     In the above-described developing device  4 , the developer supply opening  46  is provided upstream of the stirring chamber  41   b  with respect to the developer feeding direction and outside a circulating path, of the developer, formed by the developing chamber  41   a  and the stirring chamber  41   b . Specifically, the developer supply opening  46  is provided upstream of one delivering portion  41   d  with respect to the developer feeding direction of the stirring chamber  41   b . Accordingly, in the neighborhood of the developer supply opening  46 , the developer in the developer circulating path little exists, and the developer for supply only passes. 
     Such supply by the developer supplying device  80  is carried out by automatic toner replenisher (ATR) control. This ATR control is such that an operation of the developer supplying device  80  is controlled depending on an image ratio during image formation, the toner content detected by the inductance sensor  45 , and a density detection result of a patch image by a density sensor  103  ( FIG. 1 ) for detecting a density of the toner, and thus the developer is supplied (replenished) to the developing device  4 . 
     The density sensor  103  is, as shown in  FIG. 1 , provided downstream of the most downstream image forming portion PY and upstream of the secondary transfer portion T 2  with respect to the rotational direction of the intermediary transfer belt  51  so as to oppose the intermediary transfer belt  51 . In control using the density sensor  103 , for example, at timing such as the time of a start of an image forming job or every image formation of a predetermined print number, a toner image for control (patch image) is transferred onto the intermediary transfer belt  51  and the density of the patch image is detected by the density sensor  103 . Then, on the basis of this detection result, supply control of the developer by the developer supplying device  80  is carried out. 
     Incidentally, the constitution of supplying the developer to the developing device  4  is not limited to such a constitution, but a conventionally known constitution may also be employed. 
     [Scattering of Developer] 
     Here, scattering of the developer generating from the developing device  4  will be described. First, as regards the image forming apparatus, not only speed-up and image quality improvement of an output image but also simplification of maintenance are required. As one method of the simplification of maintenance, a lowering in degree of contamination of the inside of the image forming apparatus with the developer can be cited. When the inside of the image forming apparatus is contaminated with the developer, an image defect such as contamination of the output image generates, and a cleaning operation is required at the time of exchange of the developing device, the photosensitive drum or the like in some cases. Further, in the case where the developer is deposited on respective systems such as gears, there is a liability that a slip generates in the driving systems. 
     As one cause of the above-described contamination of the inside of the image forming apparatus with the developer, scattering of the developer from the inside of the developing device can be cited. For example, in the case of a two-component developer, usually, inside the developing device, the toner and the carrier are triboelectrically charged with each other, and therefore, the toner and the carrier are attracted to each other by an electrostatic force. However, there is a liability that due to some impact (shock), scattering of the developer is generated such that this attraction is released (eliminated) and the toner liberated from the carrier is discharged together with air from the inside of the developing device. 
     A specific example of the scattering of the developer will be described using a developing device  400  in a comparison example shown in  FIG. 6 . The developing device  400  has the same constitution as that of the above-described developing device  4  except that a constitution of a developing container  401  is different from the constitution of the above-described developing container  41 . For this reason, the same constituent elements will be described by adding the same reference numerals or symbols. To the developing device  400 , similarly as in the case of the above-described developing device  4 , the supplying and feeding portion  83  of the developer supplying device  80  is connected. 
     The developing container  401  includes an upper cover  402  for covering a portion above the developing sleeve  44 . Further, between the upper cover  402  and the developing sleeve  44 , a flow path of air flowing into the developing container  401  by rotation of the developing sleeve  44  is formed. This flow path opens at a position opposing the photosensitive drum  1 , so that the scattering of the developer from the inside of the developing device principally generates from this flow path. This is because on a side opposite from this flow path (on a lower side of  FIG. 6 ), the developing blade  42  is close to and opposes the developing sleeve  44 . That is, at this position, a state in which a layer thickness of the developer carried on the developing sleeve  44  is regulated by the developing blade  42  is formed, so that the air does not readily flow out from a gap between the developing sleeve  44  and the developing blade  42 . 
     Here, the scattering of the developer refers to that the developer such as liberated toner or the like generating in the developing container  401  by stirring and feeding of the developer or by supply of the developer passes through an opening of the flow path and is discharged to an outside of the developing container  401  and is not completely collected in the developing container  401 . 
     First, toner liberation will be described. The toner and the carrier which are accommodated in the developing container  401  are triboelectrically charged with each other in the stirring chamber  41   b  and the developing chamber  41   a  and are attracted to each other by an electrostatic attraction (deposition) force generated due to the triboelectric charge and by a non-electrostatic attraction force generated due to a surface property or the like. When an impact or a shearing force is exerted on the toner deposited on the carrier, the toner is peeled off the carrier and thus is liberated from the carrier in the developing container  401 . As the impact or the shearing force at this time, behavior of the developer during feeding of the developer by the developing sleeve  44  is cited. 
     The developer forms, on the developing sleeve  44 , a magnetic chain which is a chain-like structure along magnetic lines of force of inside magnetic poles. This magnetic chain raises formed with respect to the rotational direction immediately in front of the magnetic pole and falls formed with respect to the rotational direction when the magnetic chain passes through the magnetic pole. In this case, the rotational direction of the magnetic chain is the same as the rotational direction of the developing sleeve  44 . By an impact and a centrifugal force when the magnetic chain falls, the toner is peeled off the carrier. This causes toner liberation. 
     The magnetic pole largely contributing to the toner liberation when the developer is fed by the developing sleeve  44  is the peeling magnetic pole S 3  generating a repulsive magnetic field between itself and the attracting magnetic pole S 2 . At this peeling magnetic pole S 3 , in order to peel the developer off the developing sleeve  44 , a magnetic force in a direction opposite to the rotational direction of the developing sleeve  44  is applied by the magnetic pole, so that a speed of the fed developer is lowered and thus the developer is stagnated. At this time, a length of the magnetic chain increases, and therefore, there is a tendency that the impact and the centrifugal force when the magnetic chain falls become large and thus a toner liberation amount increases. 
     Further, also the developer rose into the air before being sufficiently stirred when the developer is supplied from the developer supplying device  80  to the developer supply opening  46  causes the liberated toner in the developing container  401 . The toner supplied to the developer supply opening  46  is fed while being stirred with the developer which has already existed in the stirring chamber  41   b . At this time, in a mixing region of the developer for supply and the already-existing developer, a mixing ratio between the toner and the developer temporarily increases. In the case where the mixing ratio between the toner and the developer is high, a charge amount of the toner lowers, so that an electrostatic depositing force between the toner and the carrier lowers. The toner which is not completely mixed with the developer is liberated as it is or by the impact by the feeding screws  43   a  and  43   b  during stirring and feeding of the developer, so that the liberated toner rises into the air in the developing container  401 . 
     Further, in the case where the developer device  80  from which the developer is discharged by the air pressure generated by the pump portion  81   a  is used, the air pressure is transmitted through the supplying and feeding portion  83 , so that the air flows into the developing container  401  through the developer supply opening  46  in some cases. At this time, an air stream flowing into the developing container  401  raises, into the air in the developing container  401 , the liberated toner at a portion where the mixing ratio between the developer and the toner in the neighborhood of the developer supply opening  46  is high. Further, the air pressure transmission to the developing container  401  causes unsteady rise of the atmospheric pressure from the developer supply opening  46  to the stirring chamber  41   b . This rise of the atmospheric pressure causes the flowing of the liberated toner to the outside of the developing container  401  as described later. Particularly, such inflow of the air by the supply of the developer constitutes one factor of the scattering of the developer at an end portion, including the developer supply opening  46 , with respect to a longitudinal direction of the developing container  401  (the rotational axis direction of the developing sleeve  44 ). 
     Next, using  FIG. 6 , the air stream inside and in the neighborhood of the developing device  400  will be described. The air stream is generated in the neighborhood of the developing device  400  by the developing sleeve  44  and the photosensitive drum  1  in the following manner. First, by the rotation of the developing sleeve  44  and behavior of the magnetic chain on the magnetic pole, the air stream is generated in the substantially same direction as the rotational direction of the developing sleeve  44 . This air stream generated in the substantially same direction as the rotational direction of the developing sleeve  44  takes the air into the developing container  401  through a communication opening between the inside and the outside of the developing container  401 . Further, the air flows into the developing container  401  also by the supply of the developer. 
     Assuming that the developing container  401  is a substantially closed space, the air is gas, and therefore, continuity equation is applicable. When a flow rate of the air is v and a density of the air is ρ, there is no source flow of the air in the developing container  401 , and therefore, the following formula (1) holds.
 
∂ρ/∂ t+∇ρv= 0  (1)
 
     When a steady state is considered, in respective regions in the developing container  401 , the density ρ is roughly constant and therefore, the formula (1) can be represented by the following formula (2).
 
ρ∇v=0  (2)
 
     From this formula (2), a flow rate ρv of the air is conserved. In a longitudinal cross-section in the neighborhood of the developing device  400 , income and expenditure of the flow rate ρv is 0, so that the air is discharged to the outside of the developing device  400  in the same amount as the flow rate of the air flowing into the developing container  401  by the developing sleeve  44  and the supply of the developer. Here, the flow rate of the air flowing into the developing container  401  through a communication opening, constituted by the upper cover  402  of the developing container  401  and by the developing sleeve  44 , with rotation of the developing sleeve  44  is Qa (sleeve inflow). Further, the air stream discharged through the communication opening between the inside and the outside of the developing container  401  passes through the upper cover  402  side so as to oppose the flow of the air taken through this communication opening. The flow rate of the thus discharged air stream is Qb(sleeve discharge). Further, when the flow rate of the air stream flowing into the developing container  401  with the supply of the developer to the developing device  400  is Qd(supply inflow), a relationship of the following formula (3) holds.
 
 Qa (sleeve inflow)+ Qd (supply inflow)= Qb (sleeve discharge)  (3)
 
     The air stream taken by the developing sleeve  44  and flowing along the developing sleeve  44  is turned back in the developing container  401  and then is discharged. At this time, at the developer stagnation portion of the peeling magnetic pole S 3 , when the air stream including the developer peeled off the developing sleeve  44  is turned back, the air stream moves toward a discharge direction while containing, in a large amount, the developer such as the liberated toner generated in the developing container  401 . 
     A step in which the developer contained in the sleeve discharge air (flow rate Qb) is discharged to the outside of the developing container  401  is principally constituted by the following two component steps (factors). A first component step (factor) is such that the sleeve discharge air (flow rate Qb) discharged to the outside of the developing device  400  through the communication opening is directly discharged from a gap between the upper cover  402  and the photosensitive drum  1 . A second component step (factor) is such that the sleeve discharge air (flow rate Qb) is mixed, in the neighborhood of the photosensitive drum  1 , with the developer carried on the developing sleeve  44  or the developer is transferred, by force of inertia, to an air streaming generated by rotation of the photosensitive drum  1  and is then discharged while being carried on the air stream g. 
     The scattering of the developer is caused by discharge of the developer to the outside of the developer due to at least one factor of the above-described two factors (component steps). Then, the scattered developer contaminates the periphery of the developing device  400 , an outer wall of the developing container  401 , the photosensitive drum  1 , the exposure device  3  and the transfer device  5 . 
     [Structure of Developing Container in this Embodiment] 
     Therefore, in this embodiment, the developing container  41  of the developing device  4  is constituted as follows. A detailed structure of the developing container  41  in this embodiment will be described using  FIG. 7 . Incidentally, angles θ 1  to θ 6  are angles which are based on a horizontal plane H passing through a center opening of the developing sleeve  44  and which are formed by a line segment connecting the center opening and an objective position and by a plane (vertical plane) P perpendicular to the horizontal plane H passing through the center O. 
     Further, a curve C shown at a periphery of  FIG. 7  shows a distribution of magnetic flux density of the respective magnetic poles. Further, a rotational direction of the developing sleeve  44  is R. Of the respective magnetic poles of the magnet  44   a,  with respect to the rotational direction R, the peeling magnetic pole S 3  disposed downstream of the opposing region A and the attracting magnetic pole S 3  which is disposed adjacently downstream of the peeling magnetic pole S 3  and which has the same polarity as the polarity of the peeling magnetic pole S 3  correspond to a first magnetic pole and a second magnetic pole, respectively. In  FIG. 7 , positions of the respective magnetic poles are represented by lines showing peak positions of the magnetic flux density of the respective magnetic poles. 
     The developing container  41  in this embodiment includes an upper cover  41   f  for covering the developing sleeve  44  on a side downstream of the opposing region A with respect to the rotational direction R of the developing sleeve  44 . The upper cover  41   f  includes an outer cover  47  as a first covering portion and an inner cover  48  as a second covering portion. The outer cover  47  is disposed downstream of the opposing region A with respect to the rotational direction R and covers the developing sleeve  44  with a gap. 
     The inner cover  48  is disposed between the outer cover  47  and the developing sleeve  44  so as to provide a gap between itself and the outer cover  47  and a gap between itself and the developing sleeve  44  and covers the developing sleeve  44 . A part of the inner cover  48  opposes a part of the outer cover  47  with the gap along the rotational direction R. In this embodiment, an upstream end  48   a  of the inner cover  48  with respect to the rotational direction R of the developing sleeve  44  is opposed to a part of the outer cover  47  with a gap with respect to the rotational direction R. 
     Further, the upstream end  48   a  of the inner cover  48  with respect to the rotational direction R is positioned above the developing sleeve  44  in a side downstream, with respect to the rotational direction R, a perpendicular plane (vertical plane) P passing through a top (point) of the developing sleeve  44  with respect to a vertical direction. That is, the upstream end  43   a  of the inner cover  48  is positioned downstream of a portion vertically above the top of the developing sleeve  44 . In other words, the upstream end  48   a  of the inner cover  48  is positioned on the inside (downstream side with respect to the rotational direction R) of the developing container  41  more than the perpendicular plane P passing through a center O of the developing sleeve  44 . 
     A downstream end  48   b  of the inner cover  48  with respect to the rotational direction R is positioned in a side downstream of a position of an upstream minimum M 1  of a pair of minimum M 1  and M 2 , with respect to the rotational direction R, in terms of an absolute value of a magnetic flux density distribution of the peeling magnetic pole S 3 . The downstream end  48   b  of the inner cover  48  is positioned in a side upstream of the downstream minimum M 2  with respect to the rotational direction R. 
     Incidentally, the rotational direction downstream end  48   b  of the inner cover  48  may preferably be positioned at an upstream end W 1 , with respect to the rotational direction R of the developing sleeve  44 , of a half-width W of the magnetic flux density of the peeling magnetic pole S 3  or positioned in a side downstream of the upstream end W 1  of the half-width W with respect to the rotational direction R. The rotational direction downstream end  48   b  of the inner cover  48  may more preferably be positioned at a peak position of the magnetic flux density of the peeling magnetic pole S 3  or positioned in a side downstream of the peak position with respect to the rotational direction R. By disposing the position of the rotational direction downstream end  48   b  of the inner cover  48  at a position satisfying these conditions, a range in which the peeling magnetic pole S 3  is covered with the inner cover  48  can be broadened. 
     However, the rotational direction downstream end  48   b  of the inner cover  48  may preferably be in a position of the horizontal plane H passing through the center O of the developing sleeve  44  or be positioned in a side upstream of the position of the horizontal plane H with respect to the rotational direction R. This is because when the rotational direction downstream end  48   b  of the inner cover  48  is positioned in a side further downstream of this position, the developer peeled off the developing sleeve  44  is not readily taken in the developing chamber  41   a . For this reason, in this embodiment, the rotational direction downstream end  48   b  of the inner cover  48  is positioned within the range of the half-width W of the magnetic flux density distribution of the peeling magnetic pole S 3 . 
     Specifically, the outer cover  47  is formed by being bent toward the photosensitive drum  1  so that the outer cover  47  covers the developing sleeve  44  from an upper end of a side wall  41   g , provided as a part of the developing container  41  in a side opposite from the photosensitive drum  1  with respect to the developing sleeve  44 , toward the photosensitive drum  1 . Further, the outer cover  47  includes a first opposing portion  47   a  provided in the photosensitive drum  1  side, a second opposing portion  47   b  provided in the side wall  41   g  side, a continuous portion  47   c  connecting the first opposing portion  47   a  with the second opposing portion  47   b , and a third opposing portion provided at a free end of the first opposing portion  47   a.    
     The first opposing portion  47   a  opposes the developing sleeve  44  in a side upstream, with respect to the rotational direction P of the developing sleeve  44 , of a part (the continuous portion  47   c ) opposing the rotational direction upstream end  48   a  of the inner cover  48 . The second opposing portion  47   b  opposes an intermediary portion between the upstream end  48   a  and the downstream end  48   b  of the inner cover  48  with respect to the rotational direction R. 
     The second opposing portion  47   b  is disposed outside the first opposing portion  47   a  with respect to a radial direction of the developing sleeve  44  since the inner cover  48  is disposed between itself and the developing sleeve  44 . For this reason, the continuous portion  47   c  connecting an upstream end of the second opposing portion  47   b  with respect to the rotational direction R with a downstream end of the first opposing portion  47   a  with respect to the rotational direction R is provided. The continuous portion  47   c  is formed so as to be bent from the upstream end of the second opposing portion  47  with respect to the rotational direction R toward the developing sleeve  44  side. Further, the continuous portion  47   c  opposes the rotational direction upstream end  48   a  of the inner cover  48  with a gap with respect to the rotational direction R. 
     The third opposing portion  47   d  is formed so as to be bent from the upstream end of the first opposing portion  47   a  with respect to the rotational direction R outward with respect to the radial direction of the developing sleeve  44  and opposes the surface of the photosensitive drum  1 . The third opposing portion  47   d  opposes the photosensitive drum  1  in a predetermined range with respect to the rotational direction R of the photosensitive drum  1 . 
     Next, the angles θ 1  to θ 6  will be described. The angle θ 1  is an angle from the horizontal plane H to the opening  41   h  of the developing container  41 . That is, the angle θ 1  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and an upstream end of the first opposing portion  47   a  of the outer cover  47  with respect to the rotational direction R. The angle θ 2  is an angle from the horizontal plane H to a downstream end of the first opposing portion  47   a  with respect to the rotational direction R. That is, the angle θ 2  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the downstream end of the first opposing portion  47   a  with respect to the rotational direction R. Accordingly, a range from an end of the angle θ 1  to an end of the angle θ 2  constitutes the first opposing portion  47   a . The angle θ 3  is an angle from the horizontal plane H to the rotational direction upstream end  48   a  of the inner cover  48 . That is, the angle θ 3  an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the upstream end  48   a . The angle θ 4  is an angle from the horizontal plane H to the rotational direction downstream end  48   b  of the inner cover  48 . That is, the angle θ 4  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the downstream end  48   b . Accordingly, a range from an end of the angle θ 3  to an end of the angle θ 4  constitutes the inner cover  48 . The angle θ 5  is an angle from the horizontal plane H to the photosensitive drum of the peeling magnetic pole S 3 . That is, the angle θ 5  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the peak position of the peeling magnetic pole S 3 . The angle θ 6  is an angle from the horizontal plane H to a peak position of the feeding magnetic pole N 2  disposed adjacently upstream of the peeling magnetic pole S 3  with respect to the rotational direction R. That is, the angle θ 6  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the peak position of the feeding magnetic pole N 2 . 
     In the case of this embodiment, a relationship of θ 1 &lt;θ 6 &lt;θ 2  is satisfied. That is, the first opposing portion  47   a  is formed so as to cover at least the peak position of the feeding magnetic pole N 2 . In this embodiment, the upstream end of the first opposing portion  47   a  with respect to the rotational direction R is positioned in the neighborhood of an upstream minimum of a pair of minimums, with respect to the rotational direction R, in terms of an absolute value of the magnetic flux density distribution of the feeding magnetic pole N 2 . 
     Further, a relationship of θ 2 &lt;θ 3  is satisfied, and in a range from an end of the angle θ 2  to an end of the angle θ 3 , the gap where the above-described continuous portion  47   c  opposes the upstream end  48   a  of the inner cover  48 . Further, a relationship of θ 3 &lt;θ 5 &lt;θ 4  is satisfied. That is, the inner cover  48  is formed so as to cover at least the peak position of the peeling magnetic polo S 3 . Further, the angle θ 3  is made larger than an angle (90°) formed between the perpendicular plane P and the horizontal plane H. The developing sleeve  44  has a cylindrical shape, and the perpendicular plane P passes through the top (upstream end position) of the developing sleeve  44 . Accordingly, the upstream end  48   a  of the inner cover  48  is positioned in a side downstream of the top of the developing sleeve  44  with respect to the rotational direction R. 
     Here, a gap between the first opposing portion  47   a  and the developing sleeve  44  (i.e., a gap of a region from the end of the angle θ 1  to the end of the angle θ 2 ) is referred to as a first gap (first flow path) F 1 . A gap between the inner cover  48  and the developing sleeve  44  (i.e., a gap in a region from the end of the angle θ 3  to the end of the angle θ 4 ) is referred to as a second gap (second flow path) F 2 . A gap between the second opposing portion  47   b  and the inner cover  48  is referred to as a third gap (third flow path) F 3 . 
     Further, with respect to a cross-section (radial cross-section passing through a center axis of the developing sleeve  44 ) perpendicular to the rotational direction R of the developing sleeve  44 , a minimum gap of the first gap F 1  with respect to the rotational direction R is referred to as L 1 , and a minimum cross-sectional area is referred to as A 1 . Similarly, a minimum gap of the second gap F 2  with respect to the rotational direction R is referred to as L 2 , a minimum cross-sectional area is referred to as A 2 , a minimum gap of the third gap F 3  with respect to the rotational direction R is referred to as L 3 , and a minimum cross-sectional area is referred to as A 3 . 
     In this embodiment, the first opposing portion  47   a  is formed along a peripheral surface of the developing sleeve  44 , and therefore, the gap and the cross-sectional area of the first gap F 1  are substantially the same with respect to the rotational direction R. Also the inner cover  48  is formed along the peripheral surface of the developing sleeve  44 , and therefore, the gap and the cross-sectional area of the second gap F 2  are also substantially the same with respect to the rotational direction R. On the other hand, with respect to the rotational direction R, the gap and the cross-sectional area of the third gap F 3  gradually increases from an upstream side toward a central side and gradually decreases from the central side toward a downstream side. 
     As described above, the continuous portion  47   c  connecting the second opposing portion  47   b  and the first opposing portion  47   a  is caused to oppose the rotational direction upstream end  48   a  of the inner cover  48  with the gap with respect to the rotational direction R. In this embodiment, between the continuous portion  47   c  and the upstream end  48   a  and with respect to the rotational direction R, a gap formed between the first gap F 1  and the second gap F 2  and formed between the first gap F 1  and the third gap F 3  (i.e., the gap in the region from the end of the angle θ 2  to the end of the angle θ 3 ) is referred to as a fourth gap (merging path) F 4 . That is, the fourth gap F 4  is the gap such that the second, gap F 2  and the third gap F 3  communicate with the first gap F 1 . Such a gap F 4  is formed so that a gap L 4  with respect to a cross-section perpendicular to the rotational direction R of the developing sleeve  44  becomes larger toward the downstream side of the rotational direction R. 
     Further, in a side downstream of the downstream end  48   b  of the inner cover  48  with respect to the rotational direction R, a fifth gap (branch path) F 5  is provided. The fifth gap F 5  is a gap provided downstream of the second gap F 2  and the third gap F 3  with respect to the rotational direction R and which is formed between the developing sleeve  44  and the outer cover  47  or the side wall  41   g.    
     In this embodiment, the above-described minimum gaps L 1 , L 2  and L 3  and the above-described minimum cross-sectional areas A 1 , A 2  and A 3  are caused to satisfy the following relationships.
 
 A 1≤ A 2+ A 3
 
A2≤A3
 
 L 1≤ L 2+ L 3
 
L2≤L3
 
[Air Flow around Developing Sleeve]
 
     Next, an air stream (air flow) around the developing sleeve  44  will be described using  FIGS. 8 and 9 . In this embodiment, as described above, by providing the first to fifth gaps F 1  to F 5  around the developing sleeve  44 , the air stream as shown in  FIG. 8  generates. First, in the neighborhood of the developing sleeve  44  in the first gap F 1 , an air stream a generates so as to be moved with rotation of the developing sleeve  44 , so that the air flows into the developing container  41 . By inflow of the air, an internal pressure of the developing container  41  increases, and an air stream b generates in the first opposing portion  47   a  side of the first gap F 1  so that the internal pressure is maintained in an equilibrium state from an inside toward an outside of the developing container  41 . 
     Further, in the neighborhood of the developing sleeve  4  in the second gap F 2 , an air stream c generates with movement of the magnetic chain at the peeling magnetic pole S 3  ( FIG. 7 ), and the air taken in the developing container  41  by the air stream c flows backward by air streams d and e. That is, the air stream c flowed to a side downstream of the second gap F 2  with respect to the rotational direction R is branched in the fifth gap F 5  and flows backward into the second gap F 2  and the third gap F 3 , so that the air stream d generates in the inner cover  48  side of the second gap F 2  and the air stream e generates in the third gap F 3 . 
     As described above, the toner is liberated in a large amount when the magnetic chain falls down by the peeling magnetic pole S 3 , and therefore, the thus generated liberated toner is contained in a large amount in the air stream d in the second gap F 2 . For this reason, in this embodiment, the downstream end  48   b  of the inner cover  48  is positioned downstream of the position of the upstream minimum M 1  of the peeling magnetic pole S 3  in the magnetic flux density distribution, so that at least a part of the peeling magnetic pole S 3  is covered with the inner cover  48  ( FIG. 7 ). Particularly, in this embodiment, the downstream end  48   b  of the inner cover  48  is positioned downstream of the peak position of the peeling magnetic pole S 3  with respect to the rotational direction R, and therefore, when the magnetic chain falls down by the peeling magnetic pole S 3 , most of the region in which the liberated toner generates can be covered with the inner cover  48 . 
     Further, the inner cover  48  is provided between the developing sleeve  44  and the outer cover  47 , the second gap F 2  is provided between the inner cover  48  and the developing sleeve  44 , and the third gap F 3  is provided between the inner cover  48  and the outer cover  47 . Accordingly, the air stream e generated by the back-flow of the air stream c can be formed in the third gap F 3 . The third gap F 3  is isolated from the second gap F 2  by the inner cover  48 , and therefore, the air stream e constitutes the air in which an amount of the toner liberated from the carrier as described above is small. 
     Further, the rotational direction upstream end  48   a  of the inner cover  48  opposes the continuous portion  47   c  of the outer cover  47  with the fourth gap F 4  with respect to the rotational direction R. For this reason, the air stream e passing through the third gap F 3  merges with the air stream b in the first gap F 1  through the fourth gap F 4 . At this time, as shown in  FIG. 9 , the air stream f flowing through the fourth gap F 4  as a merging path constitutes an air curtain, so that the air stream d in the second gap F 2  is liable to be returned to the flow of the air stream c. As a result, the air stream d containing the liberated toner in the large amount is not readily discharged from the developing container  41 , so that scattering of the developer can be suppressed. 
     Particularly, in this embodiment, the minimum cross-sectional area A 1  of the first gap F 1  is not more than the sum of the minimum cross-sectional area A 2  of the second gap F 2  and the minimum cross-sectional area A 3  of the third gap F 3  (A 1 ≤A 2 +A 3 ). In this embodiment, the first to fifth gaps F 1  to F 5  are formed substantially in the same shape with respect to the rotational axis direction of the developing sleeve  44 . For this reason, the above-described relationship can also be represented by a relationship such that the minimum gap (length) L 1  of the first gap F 1  is not more than the sum of the minimum gap (length) L 2  of the second gap F 2  and the minimum gap (length) L 3  of the third gap F 3  (L 1 ≤L 2 +L 3 ). Incidentally, even if each of shapes of the respective gaps are different with respect to the rotational axis direction of the developing sleeve  44 , when an average of gaps at an associated position with respect to the radial direction of the developing sleeve  44  is minimum with respect to the rotational direction R, the average of the gaps at the position may be employed as a minimum gap (length). 
     In either case, by satisfying the above-described condition, an area in which the upstream end  48   a  of the inner cover  48  and the continuous portion  47   c  oppose each other can be ensured, so that an effect of the air curtain by the air stream f can be enhanced. Incidentally, in order to enhance the effect of the air curtain, it is preferable that A 1 &lt;A 2 +A 3  (L 1 &lt;L 2 +L 3 ) is satisfied. However, even when A 1 =A 2 +A 3  (L 1 =L 2 +L 3 ) holds, A 1 &lt;A 2 +A 3 +(cross-sectional area of inner cover  48 ) or L 1  (L 2 +L 3 +(thickness of inner cover  48 ) is satisfied, and therefore, the area in which a part of the inner cover  48  and the continuous portion  47   c  oppose each other can be ensured. 
     Here, a portion, of the inner cover  48 , opposing the continuous portion  47   c  which is a part of the outer cover  48  is not limited to the upstream end  48   a . For example, even when the upstream end of the inner cover  48  with respect to the inner cover  48  is in a position (for example, a position inside the part of the outer cover  47  with respect to the radial direction) which does not oppose the part of the outer covet  47 , a downstream part of the upstream end, with respect to the rotational direction R, of the inner cover  48  may only be required to oppose the part of the outer cover  47 . Further, in this embodiment, the rotational direction upstream end  48   a  of the inner cover  48  is positioned downstream of an uppermost point of the developing sleeve  44  with respect to a direction of gravitation. However, in this case, there is a possibility that the minimum gap (length) of the second gap F 2  between the inner cover  48  and the developing sleeve  44  becomes smaller than the gap (length) of the first gap F 1 . In the case where the feeding of the magnetic chain by the developing sleeve  44  is taken into consideration, presence of a potion where the gap (length) of the second gap F 2  is extremely small is not preferable. For this reason, it is preferable that a constitution in which the upstream end  48   a  of the inner cover  48  is caused to oppose the part of the outer cover  47  is employed. 
     Further, in this embodiment, the minimum cross-sectional area A 2  of the second gap F 2  is made not more than the minimum cross-sectional area A 3  of the third gap F 2  (A 2 ≤A 3 ). As a result, pressure loss of the flow path in the third gap F 2  is made smaller than pressure loss of the flow path in the third gap F 2 . Further, a flow rate of the air stream e passing through the third gap F 3  is increased, and a flow rate of the air stream d passing through the second gap F 2  is decreased. As a result, not only the above-described effect of the air curtain can be easily obtained but also the air stream e which is the air in which the amount of the liberated toner is small can be passed through a discharge path in a larger amount than the air stream d which is the air in which the amount of the liberated toner is large, so that scattering of the developer from the developing container  41  can be suppressed. 
     Incidentally, in order to make the pressure loss of the flow path in the third gap F 2  smaller than the pressure loss of the flow path in the second gap F 2 , A 2 &lt;A 3  may preferably be satisfied. However, even when A 2 =A 3  holds, in the second gap F 2 , the air stream c opposing the air stream d exists with the rotation of the developing sleeve  44 , and therefore, the pressure loss of the flow path in the second gap F 2  becomes larger than the pressure loss of the flow path in the third gap F 3 . 
     In order to satisfy such a relationship, the minimum gap (length) L 2  of the second gap F 2  may also be made not more than the minimum gap (L) L 3  of the third gap F 2  (L 2 ≤L 3 ). The reason therefor is the same as that described in the case of A 2 ≤A 3 . Further, also in this case, L 2 &lt;L 3  may preferably be satisfied, but similarly as described above, due to the presence of the air stream c, L 2 =L 2  may also be employed. 
     However, when the minimum cross-sectional area A 3  or the minimum gap L 3  is made excessively small, there is a liability that a flow of the air stream c for talking the scattering toner in the developing container  41  is hindered and the flow rate of the air stream e extremely lowers. For this reason, the minimum gap L 2  may preferably be set at 1.5 mm-3.0 mm, and the minimum gap L 3  may preferably be set at 2.0 mm-3.5 mm. 
     Further, in the case of this embodiment, the fourth gap F 4  is disposed so as not to overlap with the peak position (end of the angle θ 6 ) of the feeding magnetic pole N 2 . That is, the fourth gap F 4  is formed at a position deviated from the peak position of the feeding magnetic pole N 2  in the rotational direction R, and in this embodiment, is disposed downstream of the peak position with respect to the rotational direction R. This is because when the fourth gap F 4  and the peak position of the feeding magnetic pole N 2  overlap with each other, the scattering toner generating when the magnetic chain of the feeding magnetic pole N 2  starts to fall down is diffused by the air stream f and thus the effect of the air curtain is lowered. 
     Further, in this embodiment, the upstream end  48   a  of the inner cover  48  is positioned downstream of a position vertically above the top (point) of the developing sleeve  44  with respect to the rotational direction R. In other words, the upstream end  48   a  of the inner cover  48  is positioned inside the developing container  41  more than the perpendicular plane P passing through the developing sleeve  44  is. The toner is liable to deposit on the upper surface of the inner cover  48  and on the upstream end  48   a . For this reason, there is a liability that the toner deposited thereon falls from the upstream end  48   a  due to some factor. Here, in the case where the deposited toner falls in a side upstream of the top of the developing sleeve  44  with respect to the rotational direction R, there is a liability that the dropped toner is deposited on the photosensitive drum  1  and has the influence on an image formed on the photosensitive drum  1 . 
     On the other hand, in this embodiment, the upstream end  48   a  of the inner cover  48  is positioned downstream of the top of the developing sleeve  44  with respect to the rotational direction R, and therefore, the toner deposited on the inner cover  48  falls from the upstream end  48   a  toward a side downstream of the top of the developing sleeve  44  with respect to the rotational direction R. Accordingly, the dropped toner is taken inside the developing container  41  with the rotation of the developing sleeve  44 , so that the influence of the dropped toner on the image formed on the photosensitive drum  1  can be suppressed. 
     Further, in the case of this embodiment, at a free end portion of the cover  47  on the photosensitive drum  1  side, the third opposing portion  47   d  opposing the photosensitive drum  1  is provided in a predetermined range with respect to the rotational direction. Further, between the third opposing portion  47   d  and the photosensitive drum  1 , a sixth gap (sixth flow path) F 6  is formed along the rotational direction of the photosensitive drum  1 . As shown in  FIG. 8 , in the sixth gap F 6 , an air stream g generates with rotation of the photosensitive drum  1 . The air stream g is a flow in a direction in which the air is discharged from the sixth gap F 6 . On the other hand, in the sixth gap F 6 , in order to make in flow and out flow of the air in the sixth gap F 6  equivalent, an air stream h flows from outside air in a direction opposite to the direction of the air stream g. 
     This air stream h constitutes the air curtain, so that the air stream b in the first gap F 1  flows into the gap between the photosensitive drum  1  and the developing sleeve  44  or is merged with the air stream a by being returned. In the case where the air stream b in the first gap F 1  flows into the gap between the photosensitive drum  1  and the developing sleeve  44 , the developer such as the liberated toner contained in the air stream b is caught by the magnetic chain carried on the developing sleeve  44 , so that the scattering of the developer can be suppressed. Further, the air stream b merges with the air stream a, so that the air stream b is not readily discharged to the outside of the developing container  41 . For this reason, in the case where the air stream b contains the developer, the scattering of the developer can be suppressed. Further, as regards the air stream g in the neighborhood of the photosensitive drum  1 , the liberated toner is deposited little by little on the photosensitive drum  1 . For this reason, it is also possible to suppress leakage-out of the toner contained in the air stream g. 
     As described above, according to the constitution of this embodiment, the developer scattering can be sufficiently suppressed. Further, even if the developer is scattered, a scattering amount is small, and therefore, even when the developer is deposited on the image, a deposition amount is to the extent such that the deposited toner cannot be visually recognized, so that a lowering in image quality can be suppressed. 
     [Comparison Experiment] 
     In order to confirm an effect of this embodiment, an experiment in which the toner scattering amount was compared between a constitution of a comparison example and the constitution of this embodiment will be described. First, an outline of a toner scattering amount measuring method employed in this experiment will be described with reference to  FIG. 7 . Incidentally, an apparatus used in the experiment is prepared by assembling the photosensitive drum, the developing device and other constituent members, excluding the exposure device, disposed at the periphery of the photosensitive drum into a unit. In the experiment, similarly as during normal image formation, in a state in which the rotation of the photosensitive drum, the drive of the charging device and the developing device and the bias application are carried out, the toner scattering amount was measured in the following manner. 
     In a region excluding both longitudinal ends of the developing device  4 , the toner in the developing device  4  passes through the sixth gap F 6  between the photosensitive drum  1  and the third opposing portion  47   d , of the outer cover  47 , opposing the photosensitive drum  1  and is scattered to the outside of the developing device  4 . Therefore, a substantially central portion of the sixth gap F 6  with respect to the longitudinal direction longitudinal direction (rotational axis direction of the developing sleeve  44 ) is irradiated with line laser beam (light) so as to be perpendicular to the developing sleeve  44  and the photosensitive member  1 . The line laser beam is a laser beam (light) which is emitted in a line shape with a certain line width and which forms a sector-shaped two-dimensional plane optical path. The line laser beam is usually prepared by scattering a dot laser beam in a certain direction by a cylindrical lens or a rod lens. The scattering toner flying on the optical path of the line laser beam scatters the laser light (beam). For that reason, from a direction substantially perpendicular to an irradiation direction of the line laser beam, a laser irradiation range is observed through a high-speed camera or the like, whereby it is possible to measure the number of particles and a locus of the scattering toner present in the laser irradiation range. 
     As regards the line laser beam, a YAG laser (“DPGL-5W”, manufactured by Japan Laser Corp.) was used as a light source. Further, an optical system using a cylindrical lens (attached to the product) was adjusted so that a line width was 0.5 mm in the sixth gap F 6  and then an object was irradiated with the line laser beam. For observation, a high-speed camera (“SA-3”, manufactured by PHOTORON Ltd.) was used. Further, in order to permit observation of the scattering toner on the line laser beam, a shooting condition (frame rate and exposure time) and the optical system (such as the lens) of the high-speed camera were selected. 
     The number of scattering (scattered) toner particles, obtained by the above-described method, passing through the substantially longitudinal central portion of the sixth gap F 6  was converted into a scattering toner (particle) number corresponding to that per A4-sized sheet (210 mm×297 mm). Incidentally, the experimental apparatus (device) was constituted as described above, and therefore, in this conversion, contribution of image region end portions, contribution of the toner supply and the influence of the air flow in the image forming apparatus on the toner scattering are not taken into consideration. 
     In the comparison experiment, experimental apparatuses (devices) having a constitution (First Embodiment, Embodiment 1) of L 2 ≤L 3  similar to that of this embodiment, a constitution (Comparison Example 1) shown in  FIG. 6 , and a constitution (Comparison Example 2) of L 2 &lt;L 3  different from that of this embodiment were prepared and were subjected to the experiment under the above-described condition. In Embodiment 1, L 2 =2 mm and L 3 =2.5 mm were set, and in Comparison Example 2, L 2 =2.5 mm and L 3 =2 mm were set. In Comparison Example 1, no cover is provided, but the distance between the developing sleeve and the upper cover  402  was set at 2.5 mm. Further, in Comparison Example 1, the third opposing portion  47   d  was not provided, but a portion of the upper cover  402  opposing the photosensitive drum  1  was irradiated with the laser beam at a substantially longitudinal central portion thereof. Other constitutions are common to this embodiment (Embodiment 1) and Comparison Examples 1 and 2. 
     A result of this experiment is shown in  FIG. 10 . First, in the case where Comparison Examples 1 and 2 are compared with each other, the scattering toner (particle) number in Comparison Example 2 was smaller than the scattering toner number in Comparison Example 1. However, compared with Comparison Example 1, the scattering toner number could not be reduced in a large amount. This is predicted because although the air stream e generates in the third gap F 3 , also the air stream d generates due to the relationship in pressure loss between the second gap F 2  and the third gap F 3  and thus an amount in which the air stream d directly carries the liberated toner, generated in the neighborhood of the S 3  pole, to the air stream g is large. 
     Next, in the case where Comparison Example 1 and Embodiment 1 were compared with each other, the scattering toner number in Embodiment 1 was made considerably smaller than the scattering toner number in Comparison Example 1. This is predicted because due to the relationship in pressure loss between the second gap F 2  and the third gap F 3 , the air stream e is larger in amount than the air stream d, and thus the number of the scattering toner (particles) contained in the air stream g is relatively decreased. From the above, in Embodiment 1 (the constitution of this embodiment), compared with Comparison Examples 1 and 2, a degree of the toner scattering could be largely decreased. 
     [Second Embodiment] 
     The Second Embodiment (Embodiment 2) will be described. Constituent elements similar to those in the First Embodiment (Embodiment 1) are represented by the same reference numerals or symbols and will be omitted from description or briefly described. In the following, a portion different from the First Embodiment will be principally described. 
     In this embodiment, the developing container  41  of the developing device  4  is constituted as follows. A detailed structure of the developing container  41  in this embodiment will be described using  FIG. 11 . Incidentally, angles θ 1  to θ 6  are angles which are based on a horizontal plane H passing through a center opening of the developing sleeve  44  and which are formed by a line segment connecting the center opening and an objective position and by a plane (vertical plane) P perpendicular to the horizontal plane H passing through the center O. 
     Further, a curve C shown at a periphery of  FIG. 11  shows a distribution of magnetic flux density of the respective magnetic poles. Further, a rotational direction of the developing sleeve  44  is R. Of the respective magnetic poles, with respect to the rotational direction R, the peeling magnetic pole S 3  disposed downstream of the opposing region A and the attracting magnetic pole S 3  which is disposed adjacently downstream of the peeling magnetic pole S 3  and which has the same polarity as the polarity of the peeling magnetic pole S 3  correspond to a first magnetic pole and a second magnetic pole, respectively. Further, a feeding magnetic pole N 2  which is disposed adjacently upstream of the peeling magnetic pole S 3  with respect to the rotational direction R and which is different in polarity from the peeling magnetic pole S 2  corresponds to a third magnetic pole. In  FIG. 11 , positions of the respective magnetic poles are represented by lines showing peak positions of the magnetic flux density of the respective magnetic poles. 
     The developing container  41  in this embodiment includes an upper cover  41   f  for covering the developing sleeve  44  on a side downstream of the opposing region A with respect to the rotational direction R of the developing sleeve  44 . The upper cover  41   f  includes an outer cover  47  as a first covering portion and an inner cover  48  as a second covering portion. The outer cover  47  is disposed downstream of the opposing region A with respect to the rotational direction R and covers the developing sleeve  44  with a gap. 
     The inner cover  48  is disposed between the outer cover  47  and the developing sleeve  44  so as to provide a gap between itself and the outer cover  47  and a gap between itself and the developing sleeve  44  and covers the developing sleeve  44 . In this embodiment, an upstream end  48   a  of the inner cover  48  with respect to the rotational direction R of the developing sleeve  44  is opposed to a part of the outer cover  47  with a gap with respect to the rotational direction R. 
     The upstream end  48   a  of the inner cover  48  is positioned upstream of the peak position magnetic flux density of the feeding magnetic pole N 2  with respect to the rotational direction R. It is preferable that the upstream end  48   a  of the inner cover  48  is positioned at an upstream end W 12 , with respect to the rotational direction of the developing sleeve  44 , of a half-width W 11  of the magnetic flux density distribution of the feeding magnetic pole N 2  or is positioned upstream of the upstream end W 12  of the half-width W 11 . By disposing the upstream end  48   a  of the inner cover  48  at such a position, a range from the peeling magnetic pole S 3 , including the magnetic flux density peak position of the feeding magnetic pole N 2 , to the feeding magnetic pole  2  can be covered with the inner cover  48 . 
     A downstream end  48   b  of the inner cover  48  with respect to the rotational direction R is positioned in a side downstream of a position of an upstream minimum M 1  of a pair of minimum M 1  and M 2 , with respect to the rotational direction R, in terms of an absolute value of a magnetic flux density distribution of the peeling magnetic pole S 3 . The downstream end  48   b  of the inner cover  48  is positioned in a side upstream of the downstream minimum M 2  with respect to the rotational direction R. 
     Incidentally, the rotational direction downstream end  48   b  of the inner cover  48  may preferably be positioned at an upstream end W 1 , with respect to the rotational direction R of the developing sleeve  44 , of a half-width W of the magnetic flux density of the peeling magnetic pole S 3  or positioned in a side downstream of the upstream end W 1  of the half-width W with respect to the rotational direction R. The rotational direction downstream end  48   b  of the inner cover  48  may more preferably be positioned at a peak position of the magnetic flux density of the peeling magnetic pole S 3  or positioned in a side downstream of the peak position with respect to the rotational direction R. By disposing the position of the rotational direction downstream end  48   b  of the inner cover  48  at a position satisfying these conditions, a range in which the peeling magnetic pole S 3  is covered with the inner cover  48  can be broadened. 
     Specifically, the outer cover  47  is formed by being bent toward the photosensitive drum  1  so that the outer cover  47  covers the developing sleeve  44  from an upper end of a side wall  41   g , provided as a part of the developing container  41  in a side opposite from the photosensitive drum  1  with respect to the developing sleeve  44 , toward the photosensitive drum  1 . Further, the outer cover  47  includes a first opposing portion  47   a  provided in the photosensitive drum  1  side a second opposing portion  47   b  provided in the side wall  41   g  side, a continuous portion  47   c  connecting the first opposing portion  47   a  with the second opposing portion  47   b , and a third opposing portion provided at a free end of the first opposing portion  47   a.    
     The first opposing portion  47   a  opposes the developing sleeve  44  in a side upstream, with respect to the rotational direction R of the developing sleeve  44 , of a part (the continuous portion  47   c ) opposing the rotational direction upstream end  48   a  of the inner cover  48 . The second opposing portion  47   b  opposes an intermediary portion between the upstream end  48   a  and the downstream end  48   b  of the inner cover  48  with respect to the rotational direction R. 
     The second opposing portion  47   b  is disposed outside the first opposing portion  47   a  with respect to a radial direction of the developing sleeve  44  since the inner cover  48  is disposed between itself and the developing sleeve  44 . For this reason, the continuous portion  47   c  connecting an upstream end of the second opposing portion  47   b  with respect to the rotational direction R with a downstream end of the first opposing portion  47   a  with respect to the rotational direction R is provided. The continuous portion  47   c  is formed so as to be bent from the upstream end of the second opposing portion  47  with respect to the rotational direction R toward the developing sleeve  44  side. Further, the continuous portion  47   c  opposes the rotational direction upstream end  48   a  of the inner cover  48  with a gap with respect to the rotational direction R. 
     The third opposing portion  47   d  is formed so as to be bent from the upstream end of the first opposing portion  47   a  with respect to the rotational direction R outward with respect to the radial direction of the developing sleeve  44  and opposes the surface of the photosensitive drum  1 . The third opposing portion  47   d  opposes the photosensitive drum  1  in a predetermined range with respect to the rotational direction R of the photosensitive drum  1 . 
     Next, the angles θ 1  to θ 6  will be described. The angle θ 1  is an angle from the horizontal plane H to the opening  41   h  of the developing container  41 . That is, the angle θ 1  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and an upstream end of the first opposing portion  47   a  of the outer cover  47  with respect to the rotational direction R. The angle θ 2  is an angle from the horizontal plane H to a downstream end of the first opposing portion  47   a  with respect to the rotational direction R. That is, the angle θ 2  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the downstream end of the first opposing portion  47   a  with respect to the rotational direction R. Accordingly, a range from an end of the angle θ 1  to an end of the angle θ 2  constitutes the first opposing portion  47   a . The angle θ 3  is an angle from the horizontal plane H to the rotational direction upstream end  48   a  of the inner cover  48 . That is, the angle θ 3  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the upstream end  48   a . The angle θ 4  is an angle from the horizontal plane H to the rotational direction downstream end  48   b  of the inner cover  48 . That is, the angle θ 4  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the downstream end  48   b . Accordingly, a range from an end of the angle θ 3  to an end of the angle θ 4  constitutes the inner cover  48 . The angle θ 5  is an angle from the horizontal plane H to the photosensitive drum of the peeling magnetic pole S 3 . That is, the angle θ 5  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the peak position of the peeling magnetic pole S 3 . The angle θ 6  is an angle from the horizontal plane H to a peak position of the feeding magnetic pole N 2  disposed adjacently upstream of the peeling magnetic pole S 3  with respect to the rotational direction R. That is, the angle θ 6  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the peak position of the feeding magnetic pole N 2 . 
     In the case of this embodiment, a relationship of θ 1 &lt;θ 6 &lt;θ 2  is satisfied. The second opposing portion  47   b  is formed so as to cover the peak position of the feeding magnetic pole N 2 . The inner cover  48  is disposed between the second opposing portion  47   b  and the developing sleeve  44 , and therefore, also the inner cover  48  covers the peak position of the feeding magnetic pole N 2 . In this embodiment, the upstream end of the second opposing portion  47   b  with respect to the rotational direction R is positioned upstream of the upstream end W 12 , with respect to the rotational direction of the developing sleeve  44 , of the half-width W 11  of the magnetic flux density distribution of the feeding magnetic pole N 2 . In this embodiment, the upstream end of the first opposing portion  47   a  with respect to the rotational direction R is positioned in the neighborhood of an upstream minimum of a pair of minimums, with respect to the rotational direction R, in terms of an absolute value of the magnetic flux density distribution of the feeding magnetic pole N 2 . 
     Further, a relationship of θ 2 &lt;θ 3  is satisfied, and in a range from an end of the angle θ 2  to an end of the angle θ 3 , the gap where the above-described continuous portion  47   c  opposes the upstream end  48   a  of the inner cover  48 . Further, a relationship of θ 3 &lt;θ 5 &lt;θ 4  is satisfied. That is, the inner cover  48  is formed so as to cover at least the peak position of the peeling magnetic pole S 3 . Further, the angle θ 3  is made smaller than an angle (90°) formed between the perpendicular plane P and the horizontal plane H. The developing sleeve  44  has a cylindrical shape, and the perpendicular plane P passes through the top (upstream end position) of the developing sleeve  44 . Accordingly, the upstream end  48   a  of the inner cover  48  is positioned in a side upstream of the top of the developing sleeve  44  with respect to the rotational direction R. 
     Here, a gap between the first opposing portion  47   a  and the developing sleeve  44  (i.e., a gap of a region from the end of the angle θ 1  to the end of the angle θ 2 ) is referred to as a first gap (first flow path) F 1 . A gap between the inner cover  48  and the developing sleeve  44  (i.e., a gap in a region from the end of the angle θ 3  to the end of the angle θ 4 ) is referred to as a second gap (second flow path) F 2 . A gap between the second opposing portion  47   b  and the inner cover  48  is referred to as a third gap (third flow path) F 3 . 
     Further, with respect to a cross-section (radial cross-section passing through a center axis of the developing sleeve  44 ) perpendicular to the rotational direction R of the developing sleeve  44 , a minimum gap of the first gap F 1  with respect to the rotational direction R is referred to as L 1 , and a minimum cross-sectional area is referred to as A 1 . Similarly, a minimum gap of the second gap F 2  with respect to the rotational direction R is referred to as L 2 , a minimum cross-sectional area is referred to as A 2 , a minimum gap of the third gap F 3  with respect to the rotational direction R is referred to as L 3 , and a minimum cross-sectional area is referred to as A 3 . 
     In this embodiment, the first opposing portion  47   a  is formed along a peripheral surface of the developing sleeve  44 , and therefore, the gap and the cross-sectional area of the first gap F 1  are substantially the same with respect to the rotational direction R. Also the inner cover  48  is formed along the peripheral surface of the developing sleeve  44 , and therefore, the gap and the cross-sectional area of the second gap F 2  are also substantially the same with respect to the rotational direction R. On the other hand, with respect to the rotational direction R, the gap and the cross-sectional area of the third gap F 3  gradually increases from an upstream side toward a central side and gradually decreases from the central side toward a downstream side. 
     As described above, the continuous portion  47   c  connecting the second opposing portion  47   b  and the first opposing portion  47   a  is caused to oppose the rotational direction upstream end  48   a  of the inner cover  48  with the gap with respect to the rotational direction R. In this embodiment, between the continuous portion  47   c  and the upstream end  48   a  and with respect to the rotational direction R, a gap formed between the first gap F 1  and the second gap F 2  and formed between the first gap F 1  and the third gap F 3  (i.e., the gap in the region from the end of the angle θ 2  to the end of the angle θ 3 ) is referred to as a fourth gap (merging path) F 4 . That is, the fourth gap F 4  is the gap such that the second gap F 2  and the third gap F 3  communicate with the first gap F 1 . Such a gap F 4  is formed so that a gap L 4  with respect to a cross-section perpendicular to the rotational direction R of the developing sleeve  44  becomes larger toward the downstream side of the rotational direction R. 
     Further, in a side downstream of the downstream end  48   b  of the inner cover  48  with respect to the rotational direction R, a fifth gap (branch path) F 5  is provided. The fifth gap F 5  is a gap provided downstream of the second gap F 2  and the third gap F 3  with respect to the rotational direction R and which is formed between the developing sleeve  44  and the outer cover  47  or the side wall  41   g.    
     In this embodiment, the above-described minimum gaps L 1 , L 2  and L 3  and the above-described minimum cross-sectional areas A 1 , A 2  and A 3  are caused to satisfy the following relationships.
 
 A 1 ≤A 2 +A 3
 
A2≤A3
 
 L 1 ≤L 2 +L 3
 
L2≤L3
 
[Air Flow around Developing Sleeve]
 
     Next, an air stream (air flow) around the developing sleeve  44  will be described using FIGS.  12  and  9 . In this embodiment, as described above, by providing the first to fifth gaps F 1  to F 5  around the developing sleeve  44 , the air stream as shown in  FIG. 12  generates. First, in the neighborhood of the developing sleeve  44  in the first gap F 1 , an air stream a generates so as to be moved with rotation of the developing sleeve  44 , so that the air flows into the developing container  41 . By inflow of the air, an internal pressure of the developing container  41  increases, and an air stream b generates in the first opposing portion  47   a  side of the first gap F 1  so that the internal pressure is maintained in an equilibrium state from an inside toward an outside of the developing container  41 . 
     Further, in the neighborhood of the developing sleeve  4  in the second gap F 2 , an air stream c generates with movement of the magnetic chain at the peeling magnetic pole S 3  ( FIG. 11 ), and the air taken in the developing container  41  by the air stream c flows backward by air streams d and e. That is, the air stream c flowed to a side downstream of the second gap F 2  with respect to the rotational direction R is branched in the fifth gap F 5  and flows backward into the second gap F 2  and the third gap F 3 , so that the air stream d generates in the inner cover  48  side of the second gap F 2  and the air stream e generates in the third gap F 3 . 
     As described above, the toner is liberated in a large amount when the magnetic chain falls down by the peeling magnetic pole S 3 , and therefore, the thus generated liberated toner is contained in a large amount in the air stream d in the second gap F 2 . For this reason, in this embodiment, the downstream end  48   b  of the inner cover  48  is positioned downstream of the position of the upstream minimum M 1  of the peeling magnetic pole S 3  in the magnetic flux density distribution, so that at least a part of the peeling magnetic pole S 3  is covered with the inner cover  48  ( FIG. 11 ). Particularly, in this embodiment, the downstream end  48   b  of the inner cover  48  is positioned downstream of the peak position of the peeling magnetic pole S 3  with respect to the rotational direction R, and therefore, when the magnetic chain falls down by the peeling magnetic pole S 3 , most of the region in which the liberated toner generates can be covered with the inner cover  48 . 
     Further, the inner cover  48  is provided between the developing sleeve  44  and the outer cover  47 , the second gap F 2  is provided between the inner cover  48  and the developing sleeve  44 , and the third gap F 3  is provided between the inner cover  48  and the outer cover  47 . Accordingly, the air stream e generated by the back-flow of the air stream c can be formed in the third gap F 3 . The third gap F 3  is isolated from the second gap F 2  by the inner cover  48 , and therefore, the air stream e constitutes the air in which an amount of the toner liberated from the carrier as described above is small. 
     Further, the rotational direction upstream end  48   a  of the inner cover  48  opposes the continuous portion  47   c  of the outer cover  47  with the fourth gap F 4  with respect to the rotational direction R. For this reason, the air stream e passing through the third gap F 3  merges with the air stream b in the first gap F 1  through the fourth gap F 4 . At this time, as shown in  FIG. 9 , the air stream f flowing through the fourth gap F 4  as a merging path constitutes an air curtain, so that the air stream d in the second gap F 2  is liable to be returned to the flow of the air stream c. As a result, the air stream d containing the liberated toner in the large amount is not readily discharged from the developing container  41 , so that scattering of the developer can be suppressed. 
     Particularly, in this embodiment, the minimum cross-sectional area A 1  of the first gap F 1  is not more than the sum of the minimum cross-sectional area A 2  of the second gap F 2  and the minimum cross-sectional area A 3  of the third gap F 3  (A 1 ≤A 2 +A 3 ). In this embodiment, the first to fifth gaps F 1  to F 5  are formed substantially in the same shape with respect to the rotational axis direction of the developing sleeve  44 . For this reason, the above-described relationship can also be represented by a relationship such that the minimum gap (length) L 1  of the first gap F 1  is not more than the sum of the minimum gap (length) L 2  of the second gap F 2  and the minimum gap (length) L 3  of the third gap F 3  (L 1 ≤L 2 +L 3 ). Incidentally, even if each shape of the respective gaps is different with respect to the rotational axis direction of the developing sleeve  44 , when an average of gaps at an associated position with respect to the radial direction of the developing sleeve  44  is minimum with respect to the rotational direction R, the average of the gaps at the position may be employed as a minimum gap (length). 
     In either case, by satisfying the above-described condition, an area in which the upstream end  48   a  of the inner cover  48  and the continuous portion  47   c  oppose each other can be ensured, so that an effect of the air curtain by the air stream f can be enhanced. Incidentally, in order to enhance the effect of the air curtain, it is preferable that A 1 &lt;A 2 +A 3  (L 1 &lt;L 2 +L 3 ) is satisfied. However, even when A 1 =A 2 +A 3  (L 1 =L 2 +L 3 ) holds, A 1 &lt;A 2 +A 3 +(cross-sectional area of inner cover  48 ) or L 1  (L 2 +L 3 +(thickness of inner cover  48 ) is satisfied, and therefore, the area in which a part of the inner cover  48  and the continuous portion  47   c  oppose each other can be ensured. 
     Further, in this embodiment, the minimum cross-sectional area A 2  of the second gap F 2  is made not more than the minimum cross-sectional area A 3  of the third gap F 2  (A 2 ≤A 3 ). As a result, pressure loss of the flow path in the third gap F 2  is made smaller than pressure loss of the flow path in the third gap F 2 . Further, a flow rate of the air stream e passing through the third gap F 3  is increased, and a flow rate of the air stream d passing through the second gap F 2  is decreased. As a result, not only the above-described effect of the air curtain can be easily obtained but also the air stream e which is the air in which the amount of the liberated toner is small can be passed through a discharge path in a larger amount than the air stream d which is the air in which the amount of the liberated toner is large, so that scattering of the developer from the developing container  41  can be suppressed. 
     Incidentally, in order to make the pressure loss of the flow path in the third gap F 2  smaller than the pressure loss of the flow path in the second gap F 2 , A 2 &lt;A 3  may preferably be satisfied. However, even when A 2 =A 3  holds, in the second gap F 2 , the air stream c opposing the air stream d exists with the rotation of the developing sleeve  44 , and therefore, the pressure loss of the flow path in the second gap F 2  becomes larger than the pressure loss of the flow path in the third gap F 3 . 
     In order to satisfy such a relationship, the minimum gap (length) L 2  of the second gap F 2  may also be made not more than the minimum gap (L) L 3  of the third gap F 2  (L 2 ≤L 3 ). The reason therefor is the same as that described in the case of A 2 ≤A 3 . Further, also in this case, L 2 &lt;L 3  may preferably be satisfied, but similarly as described above, due to the presence of the air stream c, L 2 =L 2  may also be employed. 
     However, when the minimum cross-sectional area A 3  or the minimum gap L 3  is made excessively small, there is a liability that a flow of the air stream c for taking the scattering toner in the developing container  41  is hindered and the flow rate of the air stream e extremely lowers. For this reason, the minimum gap L 2  may preferably be set at 1.5 mm-3.0 mm, and the minimum gap L 3  may preferably be set at 2.0 mm-3.5 mm. 
     Further, in the case of this embodiment, the fourth gap F 4  is disposed so as not to overlap with the peak position (end of the angle θ 6 ) of the feeding magnetic pole N 2 . That is, the fourth gap F 4  is formed at a position deviated from the peak position of the feeding magnetic pole N 2  in the rotational direction R, and in this embodiment, is disposed upstream of the peak position with respect to the rotational direction R. In this embodiment, in order to realize such arrangement, the upstream end  48   a  of the inner cover  48  is positioned at the upstream end W 12 , with respect to the rotational direction of the developing sleeve  44 , of the half-width W 11  of the magnetic flux density distribution of the feeding magnetic pole N 2  or is positioned upstream of the upstream end W 12  of the half-width W 11 . That is, the upstream end  48   a  of the inner cover  48  is positioned so as to cover the peak position of the feeding magnetic pole N 2 . This is because when the fourth gap F 4  and the peak position of the feeding magnetic pole N 2  overlap with each other, the scattering toner generating when the magnetic chain of the feeding magnetic pole N 2  starts to fall down is diffused by the air stream f and thus the effect of the air curtain is lowered. In other words, the scattering toner generating when the magnetic chain of the feeding magnetic pole N 2  starts to fall down is easily taken inside the developing container  41  in the second gap F 2  with the rotation of the developing sleeve  44 . 
     Further, in the case of this embodiment, at a free end portion of the cover  47  on the photosensitive drum  1  side, the third opposing portion  47   d  opposing the photosensitive drum  1  is provided in a predetermined range with respect to the rotational direction. Further, between the third opposing portion  47   d  and the photosensitive drum  1 , a sixth gap (sixth flow path) F 6  is formed along the rotational direction of the photosensitive drum  1 . As shown in  FIG. 12 , in the sixth gap F 6 , an air stream g generates with rotation of the photosensitive drum  1 . The air stream g is a flow in a direction in which the air is discharged from the sixth gap F 6 . On the other hand, in the sixth gap F 6 , in order to make in flow and out flow of the air in the sixth gap F 6  equivalent, an air stream h flows from outside air in a direction opposite to the direction of the air stream g. 
     This air stream h constitutes the air curtain, so that the air stream b in the first gap F 1  flows into the gap between the photosensitive drum  1  and the developing sleeve  44  or is merged with the air stream a by being returned. In the case where the air stream b in the first gap F 1  flows into the gap between the photosensitive drum  1  and the developing sleeve  44 , the developer such as the liberated toner contained in the air stream b is caught by the magnetic chain carried on the developing sleeve  44 , so that the scattering of the developer can be suppressed. Further the air stream b merges with the air stream a, so that the air stream b is not readily discharged to the outside of the developing container  41 . For this reason, in the case where the air stream b contains the developer, the scattering of the developer can be suppressed. Further, as regards the air stream g in the neighborhood of the photosensitive drum  1 , the liberated toner is deposited little by little on the photosensitive drum  1 . For this reason, it is also possible to suppress leakage-out of the toner contained in the air stream g. 
     &lt;Third Embodiment&gt; 
     The Third Embodiment will be described. Constituent elements similar to those in the First and Second Embodiments are represented by the same reference numerals or symbols and will be omitted from description or briefly described. In the following, a portion different from the First and Second Embodiments will be principally described. 
     In this embodiment, the developing container  41  of the developing device  4  is constituted as follows. A detailed structure of the developing container  41  in this embodiment will be described using  FIG. 13 . Incidentally, angles θ 1  to θ 6  are angles which are based on a horizontal plane H passing through a center opening of the developing sleeve  44  and which are formed by a line segment connecting the center opening and an objective position and by a plane (vertical plane) P perpendicular to the horizontal plane H passing through the center O. 
     Further, a curve C shown at a periphery of  FIG. 13  shows a distribution of magnetic flux density of the respective magnetic poles. Further, a rotational direction of the developing sleeve  44  is R. Of the respective magnetic poles, with respect to the rotational direction R, the peeling magnetic pole S 3  disposed downstream of the opposing region A and the attracting magnetic pole S 3  which is disposed adjacently downstream of the peeling magnetic pole S 3  and which has the same polarity as the polarity of the peeling magnetic pole S 3  correspond to a first magnetic pole and a second magnetic pole, respectively. In  FIG. 13 , positions of the respective magnetic poles are represented by lines showing peak positions of the magnetic flux density of the respective magnetic poles. 
     The developing container  41  in this embodiment includes an upper cover  41   f  for covering the developing sleeve  44  on a side downstream of the opposing region A with respect to the rotational direction R of the developing sleeve  44 . The upper cover  41   f  includes an outer cover  47  as a first covering portion and an inner cover  48  as a second covering portion. The outer cover  47  is disposed downstream of the opposing region A with respect to the rotational direction R and covers the developing sleeve  44  with a gap. 
     The inner cover  48  is disposed between the outer cover  47  and the developing sleeve  44  so as to provide a gap between itself and the outer cover  47  and a gap between itself and the developing sleeve  44  and covers the developing sleeve  44 . A part of the inner cover  48  opposes a part of the outer cover  47  with the gap along the rotational direction R. In this embodiment, an upstream end  48   a  of the inner cover  48  with respect to the rotational direction R of the developing sleeve  44  is opposed to a part of the outer cover  47  with a gap with respect to the rotational direction R. 
     Further, the upstream end  48   a  of the inner cover  48  with respect to the rotational direction R is positioned above the developing sleeve  44  in a side downstream, with respect to the rotational direction R a perpendicular plane (vertical plane) P passing through a top (point) of the developing sleeve  44  with respect to a vertical direction. That is, the upstream end  48   a  of the inner cover  48  is positioned downstream of a portion vertically above the top of the developing sleeve  44 . In other words, the upstream end  48   a  of the inner cover  48  is positioned on the inside (downstream side with respect to the rotational direction R) of the developing container  41  more than the perpendicular plane P passing through a center O of the developing sleeve  44 . 
     A downstream end  48   b  of the inner cover  48  with respect to the rotational direction R is positioned in a side upstream of a position of a downstream minimum M 2  of a pair of minimums M 1  and M 2 , with respect to the rotational direction R, in terms of an absolute value of a magnetic flux density distribution of the peeling magnetic pole S 3 . The downstream end  48   b  of the inner cover  48  is positioned in a side downstream of a peak position of the magnetic flux density of the peeling magnetic pole S 3  with respect to the rotational direction R. The downstream end  48   b  of the inner cover  48  may preferably be in a position of the horizontal plane H passing through the center O of the developing sleeve  44  or be positioned in a side downstream of the position of the horizontal plane H with respect to the rotational direction R. By disposing the position of the downstream end  48   b  of the inner cover  48  at the position satisfying these conditions, a range from the peeling magnetic pole S 3 , including a developer stagnation portion, to the attracting magnetic pole S 2  can be covered with the inner cover  48 . 
     Specifically, the outer cover  47  is formed by being bent toward the photosensitive drum  1  so that the outer cover  47  covers the developing sleeve  44  from an upper end of a side wall  41   g , provided as a part of the developing container  41  in a side opposite from the photosensitive drum  1  with respect to the developing sleeve  44 , toward the photosensitive drum  1 . Further, the outer cover  47  includes a first opposing portion  47   a  provided in the photosensitive drum  1  side, a second opposing portion  47   b  provided in the side wall  41   g  side, a continuous portion  47   c  connecting the first opposing portion  47   a  with the second opposing portion  47   b , and a third opposing portion provided at a free end of the first opposing portion  47   a.    
     The first opposing portion  47   a  opposes the developing sleeve  44  in a side upstream, with respect to the rotational direction R of the developing sleeve  44 , of a part (the continuous portion  47   c ) opposing the rotational direction upstream end  48   a  of the inner cover  48 . The second opposing portion  47   b  opposes an intermediary portion between the upstream end  48   a  and the downstream end  48   b  of the inner cover  48  with respect to the rotational direction R. 
     The second opposing portion  47   b  is disposed outside the first opposing portion  47   a  with respect to a radial direction of the developing sleeve  44  since the inner cover  48  is disposed between itself and the developing sleeve  44 . For this reason, the continuous portion  47   c  connecting an upstream end of the second opposing portion  47   b  with respect to the rotational direction R with a downstream end of the first opposing portion  47   a  with respect to the rotational direction R is provided. The continuous portion  47   c  is formed so as to be bent from the upstream end of the second opposing portion  47  with respect to the rotational direction R toward the developing sleeve  44  side. Further, the continuous portion  47   c  opposes the rotational direction upstream end  48   a  of the inner cover  48  with a gap with respect to the rotational direction R. 
     The third opposing portion  47   d  is formed so as to be bent from the upstream end of the first opposing portion  47   a  with respect to the rotational direction R outward with respect to the radial direction of the developing sleeve  44  and opposes the surface of the photosensitive drum  1 . The third opposing portion  47   d  opposes the photosensitive drum  1  in a predetermined range with respect to the rotational direction R of the photosensitive drum  1 . 
     Next, the angles θ 1  to θ 6  will be described. The angle θ 1  is an angle from the horizontal plane H to the opening  41   h  of the developing container  41 . That is, the angle θ 1  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and an upstream end of the first opposing portion  47   a  of the outer cover  47  with respect to the rotational direction R. The angle θ 2  is an angle from the horizontal plane H to a downstream end of the first opposing portion  47   a  with respect to the rotational direction R. That is. the angle θ 2  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the downstream end of the first opposing portion  47   a  with respect to the rotational direction R. Accordingly, a range from an end of the angle θ 1  to an end of the angle θ 2  constitutes the first opposing portion  47   a . The angle θ 3  is an angle from the horizontal plane H to the rotational direction upstream end  48   a  of the inner cover  48 . That is, the angle θ 3  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the upstream end  48   a . The angle θ 4  is an angle from the horizontal plane H to the rotational direction downstream end  48   b  of the inner cover  48 . That is, the angle θ 4  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the downstream end  48   b . Accordingly, a range from an end of the angle θ 3  to an end of the angle θ 4  constitutes the inner cover  48 . The angle θ 5  is an angle from the horizontal plane H to the photosensitive drum of the peeling magnetic pole S 3 . That is, the angle θ 5  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the peak position of the peeling magnetic pole S 3 . The angle θ 6  is an angle from the horizontal plane H to a peak position of the feeding magnetic pole N 2  disposed adjacently upstream of the peeling magnetic pole S 3  with respect to the rotational direction R. That is, the angle θ 6  is an angle formed between the horizontal plane H and a line segment connecting the center O of the developing sleeve  44  and the peak position of the feeding magnetic pole N 2 . 
     In the case of this embodiment, a relationship of θ 1 &lt;θ 6 &lt;θ 2  is satisfied. That is, the first opposing portion  47   a  is formed so as to cover at least the peak position of the feeding magnetic pole N 2 . In this embodiment, the upstream end of the first opposing portion  47   a  with respect to the rotational direction R is positioned in the neighborhood of an upstream minimum of a pair of minimums, with respect to the rotational direction R, in terms of an absolute value of the magnetic flux density distribution of the feeding magnetic pole N 2 . 
     Further, a relationship of θ 2 &lt;θ 3  is satisfied, and in a range from an end of the angle θ 2  to an end of the angle θ 3 , the gap where the above-described continuous portion  47   c  opposes the upstream end  48   a  of the inner cover  48 . Further, a relationship of θ 3 &lt;θ 5 &lt;θ 4  is satisfied. Further, θ 4 ≥180° may preferably be satisfied. That is, the inner cover  48  may preferably be formed so as to cover the developer stagnation portion of the peeling magnetic pole S 3 . Incidentally, the angle θ 3  is made larger than an angle (90°) formed between the perpendicular plane P and the horizontal plane H. The developing sleeve  44  has a cylindrical shape, and the perpendicular plane P passes through the top (upstream end position) of the developing sleeve  44 . Accordingly the upstream end  48   a  of the inner cover  48  is positioned in a side downstream of the top of the developing sleeve  44  with respect to the rotational direction R. 
     Here, a gap between the first opposing portion  47   a  and the developing sleeve  44  (i.e., a gap of a region from the end of the angle θ 1  to the end of the angle θ 2 ) is referred to as a first gap (first flow path) F 1 . A gap between the inner cover  48  and the developing sleeve  44  (i.e., a gap in a region from the end of the angle θ 3  to the end of the angle θ 4 ) is referred to as a second gap (second flow path) F 2 . A gap between the second opposing portion  47   b  and the inner cover  48  is referred to as a third gap (third flow path) F 3 . 
     Further, with respect to a cross-section (radial cross-section passing through a center axis of the developing sleeve  44 ) perpendicular to the rotational direction R of the developing sleeve  44 , a minimum gap of the first gap F 1  with respect to the rotational direction R is referred to as L 1 , and a minimum cross-sectional area is referred to as A 1 . Similarly, a minimum gap of the second gap F 2  with respect to the rotational direction R is referred to as L 2 , a minimum cross-sectional area is referred to as A 2 , a minimum gap of the third gap F 3  with respect to the rotational direction R is referred to as L 3 , and a minimum cross-sectional area is referred to as A 3 . 
     In this embodiment, the first opposing portion  47   a  is formed along a peripheral surface of the developing sleeve  44 , and therefore, the gap and the cross-sectional area of the first gap F 1  are substantially the same with respect to the rotational direction R. Also the inner cover  48  is formed along the peripheral surface of the developing sleeve  44 , and therefore, the gap and the cross-sectional area of the second gap F 2  are also substantially the same with respect to the rotational direction R. On the other hand, with respect to the rotational direction R, the gap and the cross-sectional area of the third gap F 3  gradually increases from an upstream side toward a central side and gradually decreases from the central side toward a downstream side. 
     As described above, the continuous portion  47   c  connecting the second opposing portion  47   b  and the first opposing portion  47   a  is caused to oppose the rotational direction upstream end  48   a  of the inner cover  48  with the gap with respect to the rotational direction R. In this embodiment, between the continuous portion  47   c  and the upstream end  48   a  and with respect to the rotational direction R, a gap formed between the first gap F 1  and the second gap F 2  and formed between the first gap F 1  and the third gap F 3  (i.e., the gap in the region from the end of the angle θ 2  to the end of the angle θ 3 ) is referred to as a fourth gap (merging path) F 4 . That is, the fourth gap F 4  is the gap such that the second gap F 2  and the third gap F 3  communicate with the first gap F 1 . Such a gap F 4  is formed so that a gap L 4  with respect to a cross-section perpendicular to the rotational direction R of the developing sleeve  44  becomes larger toward the downstream side of the rotational direction R. 
     Further, in a side downstream of the downstream end  48   b  of the inner cover  48  with respect to the rotational direction R, a fifth gap (branch path) F 5  is provided. The fifth gap F 5  is a gap provided downstream of the second gap F 2  and the third gap F 3  with respect to the rotational direction R and which is formed between the developing sleeve  44  and the outer cover  47  or the side wall  41   g.    
     In this embodiment, the above-described minimum gaps L 1 , L 2  and L 3  and the above-described minimum cross-sectional areas A 1 , A 2  and A 3  are caused to satisfy the following relationships.
 
 A 1 ≤A 2 +A 3
 
A2≤A3
 
 L 1 ≤L 2 +L 3
 
L2≤L3
 
[Air Flow around Developing Sleeve]
 
     Next, an air stream (air flow) around the developing sleeve  44  will be described using  FIGS. 14 and 9 . In this embodiment, as described above, by providing the first to fifth gaps F 1  to F 5  around the developing sleeve  44 , the air stream as shown in  FIG. 14  generates. First, in the neighborhood of the developing sleeve  44  in the first gap F 1 , an air stream a generates so as to be moved with rotation of the developing sleeve  44 , so that the air flows into the developing container  41 . By inflow of the air, an internal pressure of the developing container  41  increases, and an air stream b generates in the first opposing portion  47   a  side of the first gap F 1  so that the internal pressure is maintained in an equilibrium state from an inside toward an outside of the developing container  41 . 
     Further, in the neighborhood of the developing sleeve  4  in the second gap F 2 , an air stream c generates with movement of the magnetic chain at the peeling magnetic pole S 3  ( FIG. 13 ), and the air taken in the developing container  41  by the air stream c flows backward by air streams d and e. That is, the air stream c flowed to a side downstream of the second gap F 2  with respect to the rotational direction R is branched in the fifth gap F 5  and flows backward into the second gap F 2  and the third gap F 3 , so that the air stream d generates in the inner cover  48  side of the second gap F 2  and the air stream e generates in the third gap F 3 . 
     As described above, the toner is liberated in a large amount when the magnetic chain falls down by the peeling magnetic pole S 3 , and therefore, the thus generated liberated toner is contained in a large amount in the air stream d in the second gap F 2 . For this reason, in this embodiment, the downstream end  48   b  of the inner cover  48  is positioned downstream of the peak position of the peeling magnetic pole S 3 , so that a part of the peeling magnetic pole S 3  is covered with the inner cover  48  ( FIG. 13 ). As a result, when the magnetic chain falls down by the peeling magnetic pole S 3 , most of the region in which the liberated toner generates can be covered with the inner cover  48 . 
     Further, the inner cover  48  is provided between the developing sleeve  44  and the outer cover  47 , the second gap F 2  is provided between the inner cover  48  and the developing sleeve  44 , and the third gap F 3  is provided between the inner cover  48  and the outer cover  47 . Accordingly, the air stream e generated by the back-flow of the air stream c can be formed in the third gap F 3 . The third gap F 3  is isolated from the second gap F 2  by the inner cover  48 , and therefore, the air stream e constitutes the air in which an amount of the toner liberated from the carrier as described above is small. 
     Further, as described above, the air stream e is an air stream generated by folding back the air stream c at the developer stagnation portion of the peeling magnetic pole S 3 , and the air stream c is capable of including the developer such as the toner peeled off the developing sleeve at the developer stagnation portion of the peeling magnetic pole S 3 . For this reason, in this embodiment, the downstream end  48   b  of the inner cover  48  is positioned upstream of a position of the downstream minimum M 2  of the magnetic flux density distribution of the peeling magnetic pole S 3  and is positioned downstream of the position of the horizontal plane H with respect to the rotational direction R, so that the developer stagnation portion is covered with the inner cover  48 . That is, the developer stagnation portion of the peeling magnetic pole S 3  is limited to the inside of the second gap F 2 , so that the toner peeled off the developing sleeve at the developer stagnation portion can be caused to flow downward with respect to the direction of gravitation by the air stream c. The peeled toner caused to flow by the air stream c is easily taken in the developer accommodated in the developing container  41  and does not readily enter the third gap F 3 . Accordingly, the air stream e becomes the air in which the amount of the peeled toner is small and flows toward a discharge direction. 
     Incidentally, when the downstream end  48   b  of the inner cover  48  is positioned downstream of the downstream minimum M 2  of the pair of minimums M 1  and M 2 , depending on the amount of the developer accommodated in the developing container  41 , the downstream end  48   b  can be buried in the developer, and the developer can be caused to rise in the air by the air stream c. In order to avoid this phenomenon, the downstream end  48   b  of the inner cover  48  is positioned upstream of the downstream minimum M 2  of the magnetic flux density distribution of the peeling magnetic pole S 3 . 
     Further, the rotational direction upstream end  48   a  of the inner cover  48  opposes the continuous portion  47   c  of the outer cover  47  with the fourth gap F 4  with respect to the rotational direction R. For this reason, the air stream e passing through the third gap F 3  merges with the air stream b in the first gap F 1  through the fourth gap F 4 . At this time, as shown in  FIG. 9 , the air stream f flowing through the fourth gap F 4  as a merging path constitutes an air curtain, so that the air stream d in the second gap F 2  is liable to be returned to the flow of the air stream c. As a result, the air stream d containing the liberated toner in the large amount is not readily discharged from the developing container  41 , so that scattering of the developer can be suppressed. 
     Particularly, in this embodiment, the minimum cross-sectional area A 1  of the first gap F 1  is not more than the sum of the minimum cross-sectional area A 2  of the second gap F 2  and the minimum cross-sectional area A 3  of the third gap F 3  (A 1 ≤A 2 +A 3 ). In this embodiment, the first to fifth gaps F 1  to F 5  are formed substantially in the same shape with respect to the rotational axis direction of the developing sleeve  44 . For this reason, the above-described relationship can also be represented by a relationship such that the minimum gap (length) L 1  of the first gap F 1  is not more than the sum of the minimum gap (length) L 2  of the second gap F 2  and the minimum gap (length) L 3  of the third gap F 3  (L 1 ≤L 2 +L 3 ). Incidentally, even if each shape of the respective gaps is different with respect to the rotational axis direction of the developing sleeve  44 , when an average of gaps at an associated position with respect to the radial direction of the developing sleeve  44  is minimum with respect to the rotational direction R, the average of the gaps at the position may be employed as a minimum gap (length). 
     In either case, by satisfying the above-described condition, an area in which the upstream end  48   a  of the inner cover  48  and the continuous portion  47   c  oppose each other can be ensured, so that an effect of the air curtain by the air stream f can be enhanced. Incidentally, in order to enhance the effect of the air curtain, it is preferable that A 1 &lt;A 2 +A 3  (L 1 &lt;L 2 +L 3 ) is satisfied. However, even when A 1 =A 2 +A 3  (L 1 =L 2 +L 3 ) holds, A 1 &lt;A 2 +A 3 +(cross-sectional area of inner cover  48 ) or L 1  (L 2 +L 3 +(thickness of inner cover  48 ) is satisfied, and therefore, the area in which a part of the inner cover  48  and the continuous portion  47   c  oppose each other can be ensured. 
     Here, a portion, of the inner cover  48 , opposing the continuous portion  47   c  which is a part of the outer cover  48  is not limited to the upstream end  48   a . For example, even when the upstream end of the inner cover  48  with respect to the inner cover  48  is in a position (for example, a position inside the part of the outer cover  47  with respect to the radial direction) which does not oppose the part of the outer cover  47 , a downstream part of the upstream end, with respect to the rotational direction R, of the inner cover  48  may only be required to oppose the part of the outer cover  47 . However, in this case, there is a possibility that the minimum gap (length) of the second gap F 2  between the inner cover  48  and the developing sleeve  44  becomes smaller than the gap (length) of the first gap F 1 . In the case where the feeding of the magnetic chain by the developing sleeve  44  is taken into consideration, presence of a potion where the gap (length) of the second gap F 2  is extremely small is not preferable. For this reason, it is preferable that a constitution in which the upstream end  48   a  of the inner cover  48  is caused to oppose the part of the outer cover  47  is employed. 
     Further, in this embodiment, the minimum cross-sectional area A 2  of the second gap F 2  is made not more than the minimum cross-sectional area A 3  of the third gap F 2  (A 2 ≤A 3 ). As a result, pressure loss of the flow path in the third gap F 2  is made smaller than pressure loss of the flow path in the third gap F 2 . Further, a flow rate of the air stream e passing through the third gap F 3  is increased, and a flow rate of the air stream d passing through the second gap F 2  is decreased. As a result, not only the above-described effect of the air curtain can be easily obtained but also the air stream e which is the air in which the amount of the liberated toner and the peeled toner is small can be passed through a discharge path in a larger amount than the air stream d which is the air in which the amount of the liberated toner and the peeled toner is large, so that scattering of the developer from the developing container  41  can be suppressed. 
     Incidentally, in order to make the pressure loss of the flow path in the third gap F 2  smaller than the pressure loss of the flow path in the second gap F 2 , A 2 &lt;A 3  may preferably be satisfied. However, even when A 2 =A 3  holds, in the second gap F 2 , the air stream c opposing the air stream d exists with the rotation of the developing sleeve  44 , and therefore, the pressure loss of the flow path in the second gap F 2  becomes larger than the pressure loss of the flow path in the third gap F 3 . 
     In order to satisfy such a relationship, the minimum gap (length) L 2  of the second gap F 2  may also be made not more than the minimum gap (L) L 3  of the third gap F 2  (L 2 ≤L 3 ). The reason therefor is the same as that described in the case of A 2 ≤A 3 . Further, also in this case, L 2 &lt;L 3  may preferably be satisfied, but similarly as described above, due to the presence of the air stream c, L 2 =L 2  may also be employed. 
     However, when the minimum cross-sectional area A 3  or the minimum gap L 3  is made excessively small, there is a liability that a flow of the air stream c for taking the scattering toner in the developing container  41  is hindered and the flow rate of the air stream e extremely lowers. For this reason, the minimum gap L 2  may preferably be set at 1.5 mm-3.0 mm, and the minimum gap L 3  may preferably be set at 2.0 mm-3.5 mm. 
     Further, in the case of this embodiment, the fourth gap F 4  is disposed so as not to overlap with the peak position (end of the angle θ 6 ) of the feeding magnetic pole N 2 . That is, the fourth gap F 4  is formed at a position deviated from the peak position of the feeding magnetic pole N 2  in the rotational direction R, and in this embodiment, is disposed downstream of the peak position with respect to the rotational direction R. This is because when the fourth gap F 4  and the peak position of the feeding magnetic pole N 2  overlap with each other, the scattering toner generating when the magnetic chain of the feeding magnetic pole N 2  starts to fall down is diffused by the air stream f and thus the effect of the air curtain is lowered. 
     Further, in this embodiment, the upstream end  48   a  of the inner cover  48  is positioned downstream of a position vertically above the top (point) of the developing sleeve  44  with respect to the rotational direction R. In other words, the upstream end  48   a  of the inner cover  48  is positioned inside the developing container  41  more than the perpendicular plane P passing through the developing sleeve  44  is. That is, the toner is liable to deposit on the upper surface of the inner cover  48  and on the upstream end  48   a , and therefore, there is a liability that the toner deposited thereon falls from the upstream end  48   a  due to some factor. Here, in the case where the deposited toner falls in a side upstream of the top of the developing sleeve  44  with respect to the rotational direction R, there is a liability that the dropped toner is deposited on the photosensitive drum  1  and has the influence on an image formed on the photosensitive drum  1 . 
     In this embodiment, as described above, the upstream end  48   a  of the inner cover  48  is positioned downstream of the top of the developing sleeve  44  with respect to the rotational direction R, and therefore, the toner deposited on the inner cover  48  falls from the upstream end  48   a  toward a side downstream of the top of the developing sleeve  44  with respect to the rotational direction R. Accordingly, the dropped toner is taken inside the developing container  41  with the rotation of the developing sleeve  44 , so that the influence of the dropped toner on the image formed on the photosensitive drum  1  can be suppressed. 
     Further, in the case of this embodiment, at a free end portion of the cover  47  on the photosensitive drum  1  side, the third opposing portion  47   d  opposing the photosensitive drum  1  is provided in a predetermined range with respect to the rotational direction. Further, between the third opposing portion  47   d  and the photosensitive drum  1 , a sixth gap (sixth flow path) F 6  is formed along the rotational direction of the photosensitive drum  1 . As shown in  FIG. 14 , in the sixth gap F 6 , an air stream g generates with rotation of the photosensitive drum  1 . The air stream g is a flow in a direction in which the air is discharged from the sixth gap F 6 . On the other hand, in the sixth gap F 6 , in order to make in flow and out flow of the air in the sixth gap F 6  equivalent, an air stream h flows from outside air in a direction opposite to the direction of the air stream g. 
     This air stream h constitutes the air curtain, so that the air stream b in the first gap F 1  flows into the gap between the photosensitive drum  1  and the developing sleeve  44  or is merged with the air stream a by being returned. In the case where the air stream b in the first gap F 1  flows into the gap between the photosensitive drum  1  and the developing sleeve  44 , the developer such as the liberated toner contained in the air stream b is caught by the magnetic chain carried on the developing sleeve  44 , so that the scattering of the developer can be suppressed. Further the air stream b merges with the air stream a, so that the air stream b is not readily discharged to the outside of the developing container  41 . For this reason, in the case where the air stream b contains the developer, the scattering of the developer can be suppressed. Further, as regards the air stream g in the neighborhood of the photosensitive drum  1 , the liberated toner is deposited little by little on the photosensitive drum  1 . For this reason, it is also possible to suppress leakage-out of the toner contained in the air stream g. 
     &lt;Fourth Embodiment&gt; 
     The Fourth Embodiment will be described using  FIG. 15 . Meanings of the respective lines in  FIG. 15  are similar to those in  FIG. 11 . In the above-described embodiments, the downstream end  48   b  of the inner cover  48  was positioned upstream of the position of the horizontal plane H passing through the center O of the developing sleeve  44  with respect to the rotational direction R. On the other hand, in the case of a developing device  4 A in this embodiment, a downstream end  48 Ab of an inner cover  48 A is positioned downstream of the position of the horizontal plane H passing through the center O of the developing sleeve  44  with respect to the rotational direction R. Constitutions other than the constitution of a developing container  41 A of the developing device  4 A are similar to those in the above-described First Embodiment. Constituent elements similar to those in the above-described embodiments are represented by the same reference numerals or symbols and will be omitted from description or briefly described. In the following, a portion different from the above-described embodiments will be principally described. 
     The developing container  41 A includes an upper cover  41 Af for covering the developing sleeve  44  on a side downstream of the opposing region A with respect to the rotational direction R of the developing sleeve  44 . The upper cover  41 Af includes an outer cover  47 A as a first covering portion and an inner cover  48 A as a second covering portion. The outer cover  47 A is disposed downstream of the opposing region A with respect to the rotational direction R and covers the developing sleeve  44  with a gap. The inner cover  48 A is disposed between the outer cover  47 A and the developing sleeve  44  so as to provide a gap between itself and the outer cover  47 A and a gap between itself and the developing sleeve  44  and covers the developing sleeve  44 . 
     The outer cover  47 A includes a first opposing portion  47 Aa provided in the photosensitive drum  1  side, and a second opposing portion  47 Ab provided in the side wall  41   g  side. The first opposing portion  47 Aa opposes the developing sleeve  44  in a side upstream, with respect to the rotational direction R of the developing sleeve  44 , of a part opposing the rotational direction upstream end  48 Aa of the inner cover  48 A. The second opposing portion  47 Ab opposes an intermediary portion between the upstream end  48 Aa and the downstream end  48 Ab of the inner cover  48 A with respect to the rotational direction R. 
     In the case of this embodiment, the first opposing portion  47 Aa is formed by being bent from an end portion of the second opposing portion  47 Ab on the photosensitive drum  1  side toward the developing device  44  side, and a free end thereof is caused to oppose the developing sleeve  44  with a first gap F 1 . Further, a side surface of the first opposing portion  47 Aa opposes the photosensitive drum  1  with a sixth gap F 6  in a predetermined range along a rotational direction of the photosensitive drum  1 . 
     The position of the upstream end  48 Aa of the inner cover  48  is the same as that in the Second Embodiment. That is, the position of the upstream end  48 Aa of the inner cover  48 A is in a side upstream of a peak position of magnetic flux density of the feeding magnetic pole N 2  with respect to the rotational direction R. The downstream end  48 Ab of the inner cover  48 A is positioned in a side upstream of a position of a downstream minimum M 2  of a pair of minimums M 1  and M 2 , with respect to the rotational direction R, in terms of an absolute value of a magnetic flux density distribution of the peeling magnetic pole S 3 . The downstream end  48 Ab of the inner cover  48 A is positioned in a side downstream of a peak position of the magnetic flux density of the peeling magnetic pole S 3  with respect to the rotational direction R. The downstream end  48 Ab of the inner cover  48 A may preferably be in a position of the horizontal plane H passing through the center O of the developing sleeve  44  or be positioned in a side downstream of the position of the horizontal plane H with respect to the rotational direction R. By disposing the position of the downstream end  48 Ab of the inner cover  48 A at the position satisfying these conditions, a wide range from the peeling magnetic pole S 3 , including a developer stagnation portion, to the attracting magnetic pole S 2  can be covered with the inner cover  48 . In such a case of this embodiment, the inner cover  48 A covers the feeding magnetic pole N 2  over the peak position, and therefore, a degree of scattering of the toner liberated at the feeding magnetic pole N 2  can also be reduced. Other requirements of the respective constitutions are similar to those in the Second Embodiment. 
     &lt;Fifth Embodiment&gt; 
     The Fifth Embodiment will be described using  FIG. 16 . Meanings of the respective lines in  FIG. 16  are similar to those in  FIG. 13 . In the Third Embodiment, the upstream end  48   a  of the inner cover  48  was positioned downstream of the top of the developing sleeve  44  with respect to the rotational direction R. On the other hand, in the case of a developing device  4 A in this embodiment, an upstream end  48 Aa of an inner cover  48 A is positioned upstream of the top of the developing sleeve  44  with respect to the rotational direction R. Constitutions other than the constitution of a developing container  41 A of the developing device  4 A are similar to those in the above-described First Embodiment. Constituent elements similar to those in the First Embodiment are represented by the same reference numerals or symbols and will be omitted from description or briefly described. In the following, a portion different from the Third Embodiment will be principally described. 
     The developing container  41 A includes an upper cover  41 Af for covering the developing sleeve  44  on a side downstream of the opposing region A with respect to the rotational direction R of the developing sleeve  44 . The upper cover  41 Af includes an outer cover  47 A as a first covering portion and an inner cover  48 A as a second covering portion. The outer cover  47 A is disposed downstream of the opposing region A with respect to the rotational direction R and covers the developing sleeve  44  with a gap. The inner cover  48 A is disposed between the outer cover  47 A and the developing sleeve  44  so as to provide a gap between itself and the outer cover  47 A and a gap between itself and the developing sleeve  44  and covers the developing sleeve  44 . 
     The outer cover  47 A includes a first opposing portion  47 Aa provided in the photosensitive drum  1  side, and a second opposing portion  47 Ab provided in the side wall  41   g  side. The first opposing portion  47 Aa opposes the developing sleeve  44  in a side upstream, with respect to the rotational direction R of the developing sleeve  44 , of a part opposing the rotational direction upstream end  48 Aa of the inner cover  48 A. The second opposing portion  47 Ab opposes an intermediary portion between the upstream end  48 Aa and the downstream end  48 Ab of the inner cover  48 A with respect to the rotational direction R. 
     In the case of this embodiment, the first opposing portion  47 Aa is formed by being bent from an end portion of the second opposing portion  47 Ab on the photosensitive drum  1  side toward the developing device  44  side, and a free end thereof is caused to oppose the developing sleeve  44  with a first gap F 1 . Further, a side surface of the first opposing portion  47 Aa opposes the photosensitive drum  1  with a sixth gap F 6  in a predetermined range along a rotational direction of the photosensitive drum  1 . 
     The upstream end  48 Aa of the inner cover  48  is positioned upstream of the top of the developing sleeve  44  with respect to the rotational direction R, and in this embodiment, is positioned upstream of the peak position (end of the angle θ 6 ) of the feeding magnetic pole N 2 . On the other hand, the downstream end  48 Ab of the inner cover  48  is in a substantially overlapping position with the horizontal plane H passing through the center O of the developing sleeve  44 . The position of the downstream end  48 Ab may also be the same as that in the Third Embodiment. In such a case of this embodiment, the inner cover  48 A covers the feeding magnetic pole N 2  over the peak position, and therefore, a degree of scattering of the toner liberated at the feeding magnetic pole N 2  can also be reduced. Other requirements of the respective constitutions are similar to those in the Third Embodiment. 
     &lt;Sixth Embodiment&gt; 
     The Sixth Embodiment will be described using  FIGS. 17 and 18 . In the above-described embodiments, the gap between the photosensitive drum  1  and the third opposing portion  47   d  of the outer cover  47  was the same with respect to the longitudinal direction (rotational axis direction of the developing sleeve  44 ). On the other hand, in the case of a developing device  4 B in this embodiment, a gap between the photosensitive drum  1  and a third opposing portion  47 Bd of an outer cover  47 B is smaller in longitudinal end regions than in a longitudinal central region. Constitutions other than the constitution of a developing container  41 B of the developing device  4 B are similar to those in the above-described First Embodiment. Constituent elements similar to those in the above-described embodiments are represented by the same reference numerals or symbols and will be omitted from description or briefly described. In the following, a portion different from the above-described embodiments will be principally described. 
     The developing container  41 B includes an upper cover  41 Bf for covering the developing sleeve  44  on a side downstream of the opposing region A with respect to the rotational direction R of the developing sleeve  44 . The upper cover  41 Bf includes an outer cover  47 B as a first covering portion and an inner cover  48 B as a second covering portion. The outer cover  47 B is disposed downstream of the opposing region A with respect to the rotational direction R and covers the developing sleeve  44  with a gap. The inner cover  48 B is disposed between the outer cover  47 B and the developing sleeve  44  so as to provide a gap between itself and the outer cover  47 B and a gap between itself and the developing sleeve  44  and covers the developing sleeve  44 . 
     The outer cover  47 B includes a first opposing portion  47 Ba provided in the photosensitive drum  1  side, a second opposing portion  47 Bb, a continuous portion  47 Bc connecting the first opposing portion  47 Ba and the second opposing portion  47 Bb, and a third opposing portion  47 Bd provided at a free end of the first opposing portion  47 Ba. The first opposing portion  47 Ba opposes the developing sleeve  44  in a side upstream, with respect to the rotational direction R of the developing sleeve  44 , of a part (the continuous portion  47 Bc) opposing the rotational direction upstream end  48 Ba of the inner cover  48 B. The second opposing portion  47 Bb opposes an intermediary portion between the upstream end  48 Ba and the downstream end  48 Bb of the inner cover  48 B with respect to the rotational direction R. 
     The third opposing portion  47 Bd is formed by being bent from an upstream end of the first opposing portion  47 Ba with respect to the rotational direction R outwardly in a radial direction of the developing sleeve  44 , and opposes the surface of the photosensitive drum  1 . Further, the third opposing portion  47 Bd opposes the photosensitive drum  1  in a predetermined range along the rotational direction of the photosensitive drum  1 . 
     Here, in the neighborhood of the photosensitive drum  1  and the developing sleeve  44  with respect to the longitudinal direction, even when the toner in a small amount is liberated from the carrier, the toner is deposited on the photosensitive drum  1  to the extent that the toner is not visually recognized on the image. On the other hand, at image formable region end portions, which are longitudinal end portions of the photosensitive drum  1  and the developing sleeve  44 , and on outsides thereof, a force of toner deposition on the developing sleeve  44  is weak, and therefore, there is a possibility of the toner scattering to the outsides. Therefore, in this embodiment, the degree of the toner scattering in the neighborhood of the image formable region end portions is reduced. 
     As shown in  FIG. 17 , an image formable region (developer carrying region) of the developing sleeve  44  is referred to as B 1 . Further, of the third opposing portion  47 Bd, a region having a longitudinal length which is not less than 1/2  of a longitudinal length of the image formable region B 1  when a longitudinal center of the image formable region B 1  is taken as a center of the region is referred to as a central region B 2 . Further, of the third opposing portion  47 Bd, each region outside longitudinal ends of the central region B 2  is referred to as an end region B 3 . Each of the end regions B 3  includes at least an associated end portion of the developing sleeve  44 . 
     In this case, the end regions B 3  of the third opposing portion  47 Bb are caused to approach the photosensitive drum  1  more than the central region B 2  is. That is, in the case where a distance between the end region B 3  and the photosensitive drum  1  is L 5  and a distance between the central region B 2  and the photosensitive drum  1  is L 6 , the third opposing portion  47 Bd is formed so as to satisfy L 5 &lt;L 6 . In this embodiment, the central region B 2  was 290 mm -310 mm, and each of the end regions B 3  was 20 mm-40 mm. 
     As a result, an amount of in flow and out flow of air streams g and h in the central region B 2  in the sixth gap F 6  is larger than an amount of in flow and out flow of air streams g 2  and h 2  in each of the end regions B 3 . For this reason, the degree of the toner scattering in the end regions B 3  is reduced, so that an image defect due to the toner scattering in the image forming apparatus and contamination of the inside of the image forming apparatus with the scattered toner can be reduced. Other requirements of the respective constitutions are similar to those of the above-described embodiments. The inner cover  48 B in this embodiment was the same as the inner cover  48  in the First Embodiment, but is also applicable to the inner covers  48  and  48 A in other embodiments. 
     &lt;Seventh Embodiment&gt; 
     The Seventh Embodiment will be described using  FIG. 19 . In the above-described Sixth Embodiment, the gap between the photosensitive drum  1  and the third opposing portion  47 Bd of the outer cover  47 B was made smaller in the longitudinal end regions than in the longitudinal central region. On the other hand, in the case of a developing device  4 C in this embodiment, a length, with respect to the rotational direction of the photosensitive drum  1 , of a third opposing portion  47 Cd of an outer cover  47 C is larger in the longitudinal end regions than in the longitudinal central region. Constitutions other than the constitution of the third cover portion  47 Cd are similar to those in the above-described Fourth Embodiment. Constituent elements similar to those in the Sixth Embodiment are represented by the same reference numerals or symbols and will be omitted from description or briefly described. In the following, a portion different from the Sixth Embodiment will be principally described. 
     The developing container  41 C includes an upper cover  41 Cf for covering the developing sleeve  44  on a side downstream of the opposing region A with respect to the rotational direction R of the developing sleeve  44 . The upper cover  41 Cf includes an outer cover  47 C as a first covering portion and an inner cover  48 C as a second covering portion. 
     The outer cover  47 C includes the third opposing portion  47 Cd provided at a free end of the first opposing portion  47 Ba. The third opposing portion  47 Cd is formed by being bent from an upstream end of the first opposing portion  47 Ba with respect to the rotational direction R outwardly in a radial direction of the developing sleeve  44 , and opposes the surface of the photosensitive drum  1 . 
     In the case of this embodiment, portions (regions), of the third opposing portion  47 Cd, corresponding to the end regions B 3  ( FIG. 17 ) are referred to as first regions  471 , and a portion (region) of the third opposing portion  47 Cd, corresponding to the central region B 2  ( FIG. 17 ) is referred to as a second region  472 . Further, a length of each of the first regions  471  with respect to the rotational direction of the photosensitive drum  1  is made longer than a length of the second region  472  with respect to the rotational direction of the photosensitive drum  1 . That is, in the case where a length of the first region  471  is L 7  and a length of the second region  472  is L 8 , the third opposing portion  47 Cd is formed so as to satisfy L 7 &lt;L 8 . 
     As a result, an amount of in flow and out flow of air streams g and h in the second region  472  in the sixth gap F 6  is larger than an amount of in flow and out flow of air streams g 2  and h 2  in each of the first regions  471 . For this reason, the degree of the toner scattering in the first regions  471  is reduced, so that an image defect due to the toner scattering in the image forming apparatus and contamination of the inside of the image forming apparatus with the scattered toner can be reduced. Other requirements of the respective constitutions are similar to those of the First Embodiment. The inner cover  48 B in this embodiment was the same as the inner cover  48  in the First Embodiment, but is also applicable to the inner covers  48 ,  48 A and  48 B in other embodiments. 
     &lt;Other Embodiments&gt; 
     In the above-described embodiments, as the constitution of the developing devices, the constitution using the two-component developer containing the toner and the carrier were described. However, even in the case of using a one-component developer containing toner having a magnetic property, the present invention is applicable even when a constitution including the above-described peeling magnetic pole is employed. Further, the constitutions of the above-described embodiments can be carried out by being appropriately combined with each other. For example, the constitutions of the Third and Fourth Embodiments may also be combined with each other. That is, the length of the end region B 3  of the third opposing portion  47 Bd in the Third Embodiment with respect to the rotational direction of the photosensitive drum  1  may also be made larger than the length of the central region B 2  with respect to the rotational direction of the photosensitive drum  1 . Further, the above-described Sixth and Seventh Embodiments may be combined with each other, or the Sixth Embodiment or the Seventh Embodiment may also be combined with another embodiment. 
     Further, the present invention is also applicable to, other than the constitution in which in the developing chamber, the supply of the developer to the developing sleeve and collection of the developer from the developing sleeve are carried out as described above. For example, with reference to  FIG. 3 , even a constitution such that the developer is supplied from the developing chamber  41   a  to the developing sleeve  44  and the developer peeled off the developing device  44  is collected by the stirring chamber  41   b  is employed, the present invention is applicable thereto. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Applications Nos. 2017-068775 filed on Mar. 30, 2017, 2017-068776 filed on Mar. 30, 2017, 2017-068778 filed on Mar. 30, 2017, and 2017-068779 filed on Mar. 30, 2017, which are hereby incorporated by reference herein in their entirety.