Abstract:
Provided is a developing device that can restrain stress on a developer and improve image quality. This developing device ( 3   a - 3   d ) is provided with: a developing vessel ( 20 ) that accommodates the developer, which includes a magnetic carrier and toner; a supply transport screw ( 23   b ); a developing roller ( 31 ) that carries the developer supplied by the supply transport screw ( 23   b ); an ear cutting blade ( 32 ) that regulates layer thickness of the developer on the surface of the developing roller ( 31 ); and a magnet ( 50 ) magnetized on one surface and disposed on the upstream side of the ear cutting blade ( 32 ). The magnet ( 50 ) is disposed such that a magnetized surface ( 50   a ) faces the side of the developing roller ( 31 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of International Application No. PCT/JP2014/079725, filed Nov. 10, 2014, which claims the benefit of priority to Japanese Application No. 2014-004918, filed Jan. 15, 2014, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a developing device and to an image forming apparatus incorporating one. More particularly, the present invention relates to a developing device including a regulating member for regulating the layer thickness of developer on the surface of a developer carrying member, and to an image forming apparatus incorporating such a developing device. 
     BACKGROUND ART 
     There are conventionally known, as developing devices for developing an electrostatic latent image on a photosensitive member as an image carrying member, those adopting a one-component development method and those adopting a two-component development method. A two-component development method uses developer containing toner and magnetic carrier; this provides a stabile amount of electrostatic charge for a long period, and is suitable to obtain long service lives. For example, in a developing device adopting a two-component development method, developer containing toner and magnetic carrier is stored, and the developer is fed from a stirring member to a developing roller (developer carrying member). The developing roller has a magnet (such as a regulating pole) inside it, and by the action of this magnet (such as a regulating pole), developer is carried in the form of a magnetic brush on the surface of the developing roller. As the developing roller rotates, the developer is transported to a part of the developing roller opposite the photosensitive member. Then, out of the developer, only toner is fed to the photosensitive member, and thereby an electrostatic latent image on the photosensitive member is turned into a visible image as a toner image. 
     There is also known a developing device comprising a regulating member for regulating the layer thickness of developer to make constant the amount of developer that, as a developing roller rotates, is transported to a part thereof opposite a photosensitive member and a magnet arranged on the upstream side of the regulating member with respect to the rotation direction of the developing roller. As such a developing device, a developing device is known that uses a one-component developer containing magnetic toner (e.g., Patent Document 1 identified below). In this developing device, a magnet is arranged on the upstream side of a regulating member with respect to the rotation direction of a developing roller so that, at a tip part (a developing roller side tip part) of the regulating member, a magnetic pole of the polarity opposite to that of a regulating pole is induced, and thereby the developer passing between the developing roller and the regulating member is regulated to have a predetermined layer thickness. 
     LIST OF CITATIONS 
     Patent Literature 
     Patent Document 1: Japanese Patent Application Publication No. 2003-255710 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, if the above-mentioned developing device where a magnet is arranged on the upstream side of a regulating member with respect to the rotation direction of a developing roller is applied as it is to a developing device adopting two-component development, certainly a thin film of developer (a magnetic brush) can be formed stably but, inconveniently, the developer is subject to great stress, resulting in degraded image quality. Specifically, as shown in  FIG. 7 , in the magnetic field of the magnet  150  arranged on the upstream side (in  FIG. 7 , the left side) of the regulating member  132  with respect to the rotation direction of the developing roller  131 , toner is stirred and transported by a stirring/transporting member  123   b . Meanwhile, in the magnetic field, while magnetic carrier tends to link together, toner is moved. Thus, the magnetic toner and the toner rub against each other, causing an additive to be embedded in toner or to move from toner to carrier, degrading the toner and the carrier. This results in degraded image quality. 
     Devised against the background discussed above, the present invention aims to provide a developing device that can suppress stress on developer and thereby improve image quality, and to provide an image forming apparatus incorporating such a developing device. 
     Means for Solving the Problem 
     According to one aspect of the present invention, a developing device includes: a developer container for storing developer containing magnetic carrier and toner; a stirring/transporting member for stirring and transporting the developer inside the developer container; a developer carrying member for carrying the developer fed from the stirring/transporting member; a regulating member arranged opposite the developer carrying member, for regulating the layer thickness of the developer on the surface of the developer carrying member; and a magnet arranged on the upstream side of the regulating member with respect to the rotation direction of the developer carrying member, and magnetized on one face with an S pole and an N pole extending in the rotation axis direction of the developer carrying member. Here, the magnet is arranged with its magnetized face facing the developer carrying member. 
     Advantageous Effects of the Invention 
     According to the present invention, the magnet magnetized on one face is arranged opposite the developer carrying member with the magnetized face of the magnet facing the developer carrying member. This helps suppress lines of magnetic force emanating from the face of the magnet opposite from the developer carrying member, and thus helps suppress a magnetic field formed by the magnet elsewhere than on the developer carrying member side of the magnet. This helps suppress stress on developer resulting from magnetic carrier and toner rubbing against each other elsewhere than on the developer carrying member side of the magnet, leading to improved image quality. 
     Further features and advantages of the present disclosure will become apparent from the description of embodiments given below. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view schematically showing a construction of an image forming apparatus provided with a developing device according to one embodiment of the present invention; 
         FIG. 2  is a side sectional view showing a structure of a developing device according to one embodiment of the present invention; 
         FIG. 3  is an enlarged sectional view showing a structure of and around a regulating pole and a magnet in a developing device according to one embodiment of the present invention; 
         FIG. 4  is a side sectional view showing a magnetic flux distribution of magnetic poles on a developing roller in a developing device according to one embodiment of the present invention; 
         FIG. 5  is an enlarged sectional view illustrating a structure of a magnet in a developing device according to one embodiment of the present invention; 
         FIG. 6  is an enlarged plan view illustrating a structure of a magnet in a developing device according to one embodiment of the present invention; and 
         FIG. 7  is a sectional view showing one example of a conventional developing device in which a magnet is arranged on the upstream side of a regulating member with respect to the rotation direction of a developing roller. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. 
     With reference to  FIGS. 1 to 6 , a construction of an image forming apparatus  100  provided with developing devices  3   a  to  3   d  according to one embodiment of the present invention will be described. In the image forming apparatus  100  (here, a color printer), inside its body, four image forming portions Pa, Pb, Pc, and Pd are arranged in this order from the upstream side (in  FIG. 1 , the right side) with respect to the transport direction. These image forming portions Pa to Pd are provided to correspond to four different colors (cyan, magenta, yellow, and black), and respectively form a cyan, a magenta, a yellow, and a black image sequentially each through the processes of electrostatic charting, light exposure, image development, and image transfer. 
     In the image forming portions Pa to Pd, there are arranged photosensitive drums (image carrying members)  1   a ,  1   b ,  1   c , and  1   d , respectively, which carry visible images (toner images) of the different colors, and next to the image forming portions Pa to Pd, there is arranged an intermediary transfer belt  8  which is driven by driving means (unillustrated) to rotate clockwise in  FIG. 1 . The toner images formed on the photosensitive drums  1   a  to  1   d  are primarily transferred to the intermediary transfer belt  8  which moves while being kept in contact with the photosensitive drums  1   a  to  1   d  so as to be superimposed on each other. Thereafter, the toner images transferred to the intermediary transfer belt  8  are secondarily transferred by the action of a secondary transfer roller  9  to transfer paper P as one example of a recording medium. The transfer paper P having the toner images secondarily transferred to it is then, after the toner images are fixed in a fixing portion  13 , discharged out of the body of the image forming apparatus  100 . While the photosensitive drums  1   a  to  1   d  are rotated counter-clockwise in  FIG. 1 , an image formation process is performed with respect to each of the photosensitive drums  1   a  to  1   d.    
     Transfer paper P, to which toner images are to be secondarily transferred, is contained in a sheet cassette  16  arranged in a lower part of the body of the image forming apparatus  100 , and is transported via a sheet feed roller  12   a  and a registration roller pair  12   b  to a nip portion between the secondary transfer roller  9  and a driving roller  11 , described later, of the intermediary transfer belt  8 . As the intermediary transfer belt  8 , a sheet of a dielectric resin is used, and typically it is a belt with no seam (seamless belt). On the downstream side of the secondary transfer roller  9 , there is arranged a blade-shaped belt cleaner  19  for removing toner and the like left on the surface of the intermediary transfer belt  8 . 
     Next, the image forming portions Pa to Pd will be described. Around and under the photosensitive drums  1   a  to  1   d , which are arranged so as to be freely rotatable, there are arranged chargers  2   a ,  2   b ,  2   c , and  2   d  for electrostatically charging the photosensitive drums  1   a  to  1   d , an exposing device  5  for exposing the photosensitive drums  1   a  to  1   d  to light carrying image information, developing devices  3   a ,  3   b ,  3   c , and  3   d  for forming toner images on the photosensitive drums  1   a  to  1   d , and cleaning portions  7   a ,  7   b ,  7   c , and  7   d  for removing developer (toner) and the like left on the photosensitive drums  1   a  to  1   d.    
     When image data is received from a host device such as a personal computer, first, the chargers  2   a  to  2   d  electrostatically charge the surface of the photosensitive drums  1   a  to  1   d  uniformly, and then the exposing device  5  radiates light according to the image data, so that electrostatic latent images based on the image data are formed on the photosensitive drums  1   a  to  1   d . The developing devices  3   a  to  3   d  are charged with predetermined amounts of two-component developer containing cyan, magenta, yellow, and black toner respectively. When the proportion of toner in the two-component developer contained in any of the developing devices  3   a  to  3   d  falls below a prescribed value, toner is supplied to the developing device  3   a  to  3   d  from the corresponding one of the toner containers (supplying portions)  4   a  to  4   d . The toner in the developer is fed by the developing devices  3   a  to  3   d  onto the photosensitive drums  1   a  to  1   d , and electrostatically attaches to them, and thereby toner images corresponding to the electrostatic latent images formed by exposure to light from the exposing device  5  are formed. 
     Then, primary transfer rollers  6   a  to  6   d  apply a predetermined transfer voltage between the primary transfer rollers  6   a  to  6   d  and the photosensitive drums  1   a  to  1   d , so that the cyan, magenta, yellow, and black toner on the photosensitive drums  1   a  to  1   d  is primarily transferred to the intermediary transfer belt  8 . These images of four colors are formed in a predetermined positional relationship relative to each other that is prescribed for the formation of a predetermined full-color image. Thereafter, in preparation for the subsequent formation of new electrostatic latent images, the toner and the like left on the surfaces of the photosensitive drums  1   a  to  1   d  after primary transfer are removed by the cleaning portions  7   a ,  7   b ,  7   c , and  7   d.    
     The intermediary transfer belt  8  is wound around and between a driven roller  10  on the upstream side and a driving roller  11  on the downstream side. As the intermediary transfer belt  8  starts to rotate clockwise as a result of the driving roller  11  being rotated by a drive motor (unillustrated), transfer paper P is transferred, with predetermined timing, from the registration roller pair  12   b  to the nip portion (secondary transfer nip portion) between the driving roller  11  and the secondary transfer roller  9  arranged next to it, so that the full-color image on the intermediary transfer belt  8  is secondarily transferred to the transfer paper P. The transfer paper P having the toner images secondarily transferred to it is transported to the fixing portion  13 . 
     The transfer paper P transferred to the fixing portion  13  is heated and pressed by a fixing roller pair  13   a , so that the toner images are fixed to the surface of the transfer paper P, thereby forming the predetermined full-color image. The transfer paper P having the full-color image formed on it is forwarded in one of different transport directions by a branch portion  14  which branches into a plurality of directions. When an image is formed only on one side of the transfer paper P, the transfer paper P is as it is discharged onto a discharge tray  17  by a discharge roller pair  15 . 
     On the other hand, when images are formed on both sides of the transfer paper P, part of the transfer paper P having passed through the fixing portion  13  is first stuck out of the apparatus from the discharge roller pair  15 . Then, after the tail edge of the transfer paper P has passed through the branch portion  14 , the discharge roller pair  15  is rotated in the reverse direction, and the branch portion  14  is switched to another transport direction. Now, the transfer paper P is, from its tail edge, forwarded into a sheet transport passage  18 , and is transported once again, this time with the image side reversed, to the secondary transfer nip portion. Then, the next toner images formed on the intermediary transfer belt  8  are secondarily transferred to the side of the transfer paper P where no image has been formed yet. The transfer paper P having the toner images secondarily transferred to it is transported to the fixing portion  13 , where the toner images are fixed, and is then discharged onto the discharge tray  17 . 
     Next, with reference to  FIG. 2 , the structure of the developing device  3   a  will be described in detail.  FIG. 2  is a view from behind what is shown in  FIG. 1 , and the arrangement of the members in the developing device  3   a  in  FIG. 2  is the other way around in the left/right direction relative to that in  FIG. 1 . While the following description deals with the developing device  3   a  arranged in the image forming portion Pa shown in  FIG. 1 , the developing devices  3   b  to  3   d  arranged in the image forming portions Pb to Pd have basically the same structure, and therefore no overlapping description will be repeated. 
     As shown in  FIG. 2 , the developing device  3   a  has a developer container  20  formed of resin for containing two-component developer (hereinafter referred to simply as developer), and the developer container  20  is divided into a stirring/transporting compartment  21  and a feeding/transporting compartment  22  by a partition wall  20   a . In the stirring/transporting compartment  21  and the feeding/transporting compartment  22 , there are rotatably arranged a stirring/transporting screw (stirring/transporting member)  23   a  and a feeding/transporting screw (stirring/transporting member)  23   b , respectively, for mixing toner (positively charged toner) fed from the toner container  4   a  (see  FIG. 1 ) with carrier, stirring the mixture, and electrostatically charging the toner. The stirring/transporting screw  23   a  transports the developer to one side of a developing roller  31 , described later, with respect to its axial direction (the direction perpendicular to the plane of  FIG. 2 ). The feeding/transporting screw  23   b , while transporting the developer in the opposite direction to the stirring/transporting screw  23   a , feeds the developer to the developing roller  31 . 
     In opposite end parts, with respect to its length direction (the direction perpendicular to the plane of  FIG. 2 ), of the partition wall  20   a  separating the stirring/transporting compartment  21  and the feeding/transporting compartment  22  from each other, communicating portions (unillustrated) are respectively provided through which the stirring/transporting compartment  21  and the feeding/transporting compartment  22  communicate with each other in their respective end parts. 
     Thus, the developer is, while being stirred, transported in the axial direction (in the direction perpendicular to the plane of  FIG. 2 ) by the stirring/transporting screw  23   a  and the feeding/transporting screw  23   b  so as to circulate between the stirring/transporting compartment  21  and the feeding/transporting compartment  22  through the communicating portions formed in opposite end parts of the partition wall  20   a . That is, inside the developer container  20 , a circulation path for the developer is formed via the stirring/transporting compartment  21 , one communicating portion, the feeding/transporting compartment  22 , and the other communicating portion. 
     The developer container  20  extends obliquely toward the upper right corner of the  FIG. 2 , and inside the developer container  20 , a developing roller (developer carrying member)  31  is arranged over the feeding/transporting screw  23   b . The developing roller  31  is, at a side thereof facing the opening in the developer container  20  (in  FIG. 2 , the right side), opposite the photosensitive drum  1   a  and, in this region where the two components are opposite each other, feeds toner to the photosensitive drum  1   a . The developing roller  31  rotates about its rotation axis in the counter-clockwise direction in  FIG. 2 . 
     In the stirring/transporting compartment  21 , an unillustrated toner concentration sensor is arranged so as to face the stirring/transporting screw  23   a , and based on the result of detection by the toner concentration sensor, toner is supplied from the toner container  4   a  via an unillustrated toner supply port into the stirring/transporting compartment  21 . As the toner concentration sensor, a magnetic permeability sensor is used which detects the magnetic permeability of the two-component developer containing toner and magnetic carrier in the developer container  20 . 
     The developing roller  31  is composed of a cylindrical non-magnetic developing sleeve  31   a , which rotates in the counter-clockwise direction in  FIG. 2 , and a fixed magnet member  31   b  having a plurality of magnetic poles, which is placed inside the developing sleeve  31   a . In this embodiment, the fixed magnet member  31   b  has the following five magnetic poles; a regulation pole (trim pole)  42 , which is an N pole; a transport pole  43 , which is an S pole; a main pole  44 , which is an N pole; a transport pole  45 , which is an S pole; and a separation pole  46 , which is an N pole. 
     As shown in  FIG. 3 , the regulating pole  42  is arranged opposite a trimming blade  32  or a magnet  50 , both described later. The magnetic poles of the developing roller  31  have a magnetic flux distribution as shown in  FIG. 4 . The magnetic force of the regulating pole  42  expands to reach the upstream side of the magnet  50 , described later, with respect to the rotation direction of the developing roller  31  (hereinafter referred to simply as the upstream side), and the regulating pole  42  functions also as a pump-up pole for pumping up developer from the feeding/transporting screw  23   b  to the developing roller  31 . The regulation pole  42  has a magnetic force of 35 mT, and the main pole  44  has a magnetic force of 100 mT. In  FIG. 4 , the magnetic flux distribution is indicated by thick lines. 
     The developer container  20  is fitted with, along the length direction of the developing roller  31  (the direction perpendicular to the plane of  FIG. 2 ), a trimming blade (regulating member)  32  which regulates the thickness of the developer carried on the developing roller  31 . The trimming blade  32  is located on the upstream side of the position where the developing roller  31  and the photosensitive drum  1   a  are opposite each other with respect to the rotation direction (in  FIG. 2 , the counter-clockwise direction) of the developing roller  31 . Between a tip part of the trimming blade  32  and the surface of the developing roller  31 , a small interval (gap) is left. 
     The trimming blade  32  is formed of a magnetic material (such as SUS430), and is formed to have a thickness of about 1.5 mm. The trimming blade  32  is fixed to a bottom part of the developer container  20 . On the upstream side of the trimming blade  32 , a regulation upstream member  33  is arranged. The regulation upstream member  33  has a pre-regulating function, for regulating the thickness of the developer carried on the developing roller  31  to a certain degree prior to its being regulated by the trimming blade  32 , and a stress reducing function, for reducing stress on the developer 
     As shown in  FIG. 3 , on the upstream side of a tip part (a developing roller  31  side part) of the trimming blade  32  (a part thereof facing the developing roller  31 ), a magnet  50  is arranged which is, for example, a magnet sheet with a thickness of about 0.6 mm. The magnet  50  is fixed to the top face of the regulation upstream member  33 . 
     As shown in  FIG. 5 , a downstream-side end part (a trimming blade  32  side end part) of the magnet  50  is magnetized with an N pole, and thus it induces an S pole in a tip end part (a developing roller  31  side end part) of the trimming blade  32 . Accordingly, a magnetic field is produced between the tip end part of the trimming blade  32  and the developing sleeve  31   a  (regulation pole  42 ), and this gives a predetermined layer thickness to the developer that passes between the trimming blade  32  and the developing roller  31 . In  FIG. 5 , the lines of magnetic force of the magnet  50  are indicated by thick lines. 
     With the magnetic field between the magnet  50  and the developing roller  31 , the developer on the surface of the developing roller  31  is uniformized. This helps suppress uneven density in phase with the rotation pitch of the feeding/transporting screw  23   b.    
     The magnet  50  is magnetized only on one face (magnetized face  50   a ), and no lines of magnetic force emanate from the face (non-magnetized face) of the magnet  50  opposite from the magnetized face  50   a . The magnet  50  is arranged opposite the developing roller  31  with the magnetized face  50   a  facing the developing roller  31 . The magnetized face  50   a  is magnetized with two pairs of N and S poles at a pitch of about 2.0 mm in a direction along the rotation direction of the developing roller  31 . As shown in  FIG. 6 , the N and S poles extend in the rotation axis direction of the developing roller  31  (the direction perpendicular to the plane of  FIG. 2 ; the up/down direction in  FIG. 6 ). The magnet  50  has a magnetic force of 40 mT. The magnet  50  has only to be magnetized with at least one pair of N and S poles. 
     The feeding/transporting screw  23   b  described above is arranged in the vicinity of the magnet  50 . In other words, the distance from the magnet  50  to the feeding/transporting screw  23   b  is approximately equal to the distance from the magnet  50  to the regulation pole  42 . Here, the vicinity of the magnet  50  is within a distance L of the magnet  50 , L representing the distance over which the magnet  50  exerts a magnetic force (e.g., the distance from the magnet  50  to the regulation pole  42 ). 
     A direct-current voltage (hereinafter referred to as Vslv(DC)) and an alternating-current voltage (hereinafter referred to as Vslv(AC)) are applied to the developing roller  31 . These DC and AC voltages are applied to the developing roller  31  from a developing bias power supply via a bias control circuit (neither illustrated). 
     As mentioned previously, the stirring/transporting screw  23   a  and the feeding/transporting screw  23   b  circulate the developer, while stirring it, through the stirring/transporting compartment  21  and the feeding/transporting compartment  22  inside the developer container  20 , thereby electrostatically charging the toner in the developer. The developer inside the feeding/transporting compartment  22  is transported by the feeding/transporting screw  23   b  to the developing roller  31 . Then, on the developing roller  31 , a magnetic brush (unillustrated) is formed. The magnetic brush on the developing roller  31  has its thickness regulated by the trimming blade  32  and the regulation pole  42 , and is then transported, by the rotation of the developing roller  31 , to where the developing roller  31  and the photosensitive drum  1   a  is opposite each other. Due to Vslv(DC) an Vslv(AC) being applied to the developing roller  31 , the potential difference from the photosensitive drum  1   a  causes toner to fly from the developing roller  31  to the photosensitive drum  1   a , and thereby the electrostatic latent image on the photosensitive drum  1   a  is developed. 
     The toner left unused after development is transported on by the rotation of the developing sleeve  31   a , and the developer on the surface of the developing sleeve  31   a  is given a repellent magnetic pole by the separation pole  46  and the regulation pole  42 , the two poles having the same polarity. Thus, around the midpoint between the separation pole  46  and the regulation pole  42 , the developer is separated front developing sleeve  31   a , and drops into the feeding/transporting compartment  22 . The developer is then stirred and transported by the stirring/transporting screw  23   a  and the feeding/transporting screw  23   b ; then as a two-component developer that has an adequate toner concentration and that is electrostatically charged uniformly, the developer once again forms a magnetic brush on the developing sleeve  31   a  by the action of the pump-up pole (regulation pole  42 ), and is then transported to the trimming blade  32 . 
     In this embodiment, as described above, the magnet  50  is arranged opposite the developing roller  31  with the magnetized face  50   a  facing the developing roller  31 . This helps suppress lines of magnetic force emanating from the face of the magnet  50  opposite from the developing roller  31 , and thus helps suppress a magnetic field formed by the magnet  50  elsewhere than on the developing roller  31  side of the magnet  50 . This helps suppress stress on the developer resulting from magnetic carrier and toner rubbing against each other elsewhere than on the developing roller  31  side of the magnet  50  around it, leading to improved image quality. In a magnetic field, magnetic carrier tends to link together; thus, when toner is moved in magnetic carrier, they rub against each other. This causes an additive to be embedded in toner or to move from toner to carrier, leading to degraded image quality. 
     Moreover, as described above, the feeding/transporting screw  23   b  is arranged in the upstream-side vicinity of the magnet  50 . In this case, if lines of magnetic force emanate also from the face of the magnet  50  opposite from the developing roller  31 , a magnetic field is formed also in the upstream-side vicinity of the magnet  50 . Thus, toner is moved in the magnetic field by the feeding/transporting screw  23   b , and this makes the developer particularly susceptible to stress. Thus, in a case where the feeding/transporting screw  23   b  is arranged in the upstream-side vicinity of the magnet  50 , it is especially effective to apply the present invention. 
     Moreover, as described above, in the developing device  3   a  where the regulation pole  42  also serves to pump up developer from the feeding/transporting screw  23   b  to the developing roller  31 , the magnetic field by the regulation pole  42  expands to reach the feeding/transporting screw  23   b , and this makes the developer susceptible to stress. Thus, in a case were the regulation pole  42  serves also to pump up developer, it is especially effective to apply the present invention. 
     Moreover, as described above, the magnetic force of the regulation pole  42  expands to reach the upstream side of the magnet  50 . This makes it easy to pump up, with the regulation pole  42 , developer from the feeding/transporting screw  23   b  to the developing roller  31 . 
     Moreover, as described above, the magnetized face  50   a  of the magnet  50  is magnetized with two pairs of S and N poles alternately in a direction along the rotation direction of the developing roller  31 . This, compared with the magnetized face  50   a  being magnetized with one pair of S and N poles, permits the developer on the surface of the developing roller  31  to be made more uniform by the magnetic field between the magnet  50  and the developing roller  31 , and thus helps suppress uneven density in phase with the pitch of the feeding/transporting screw  23   b.    
     It should be understood that the embodiment described above is in every aspect merely illustrative and not restrictive. The scope of the present invention is defined not by the description of the embodiment given above but by the appended claims, and encompasses any modifications made in the spirit and scope equivalent to those of the claims. 
     For example, although the embodiment deals with a case where the present invention is applied to a tandem-type color image forming apparatus as shown in  FIG. 1 , this is not meant to limit the present invention. Needless to say, the present invention finds applications in a variety of image forming apparatuses, such as monochrome copiers, monochrome printers, digital multifunction peripherals, and facsimile machines, that incorporate a developing device including a regulating member for regulating the layer thickness of the surface of a developer carrying member. 
     Although the embodiment described above deals with an example where a developing roller is provided as the developer carrying member, this is not meant to limit the present invention; as the developer carrying member, a magnetic roller can be provided between the stirring/transporting member and the developing roller. 
     Although the embodiment described above deals with an example where the feeding/transporting screw and the regulating member are arranged under the developing roller, this is not meant to limit the present invention; the feeding/transporting screw and the regulating member can be arranged over, or by the side of, the developing roller. 
     Although the embodiment described above deals with an example where a regulating member formed of a magnetic material is used, this is not meant to limit the present invention; a regulating member composed of a magnet can instead be used. 
     Any appropriate combination of one or more features from the embodiment described above and from any modified example falls within the technical scope of the present invention.