Patent ID: 7787807

Claim:
A developing unit comprising: a development roller that is opposed to an image carrier and supplies toner to the image carrier; and a magnetic roller that is opposed to the development roller and supplies toner to the development roller; wherein a sleeve of the development roller and a sleeve of the magnetic roller are driven to rotate in the same circumferential direction; a first magnet member is provided on a roller shaft in the sleeve of the development roller; a second magnet member is provided on a roller shaft in the sleeve of the magnetic roller; the first and second magnet members are opposed to each other so that opposite polarities face each other, and are supported non-rotatably in circumferential directions; the first magnet member is supported so that the peak of magnetic force of the first magnet member is positioned upstream in the direction of rotation of the sleeve of the development roller from the straight line connecting the center of the roller shaft of the development roller to the center of the roller shaft of the magnetic roller; the second magnet member is provided between third and fourth magnet members having a polarity opposite to the polarity of the second magnet member; a cross-section of the second to fourth magnet members has a generally sector-like shape; and the first magnet member is a bar magnet having a generally rectangular cross-section, wherein the first magnet member has a height measured parallel to a radius of the development roller that is smaller than a radial length of the second to fourth magnet members, and the first magnet member has a width measured perpendicular to the height that is smaller than the length of the arc of the second to fourth magnet members.