Patent ID: 7953353

Claim:
A developing device comprising: a magnet roll magnetized with a plurality of magnetic poles including a main magnetic pole which are arranged in a tangential direction; a sleeve in a cylindrical shape which is arranged so as to be opposed to an image holding body having an endless peripheral face on which a latent image is formed by a difference in electrostatic potential and to which developing agent is supplied to an outer circumference face of the sleeve; and a supporting frame that stably supports the magnet roll in a manner of restraining the magnet roll from rotating in the tangential direction, and supports the sleeve so as to rotate in the tangential direction around the magnet roll, wherein the magnet roll is so arranged that the main magnetic pole is opposed to the image holding body, and a point on the peripheral face where magnetic flux density of the main magnetic pole in the tangential direction at a position along the peripheral face of the magnet roll is zero is set at a position predetermined with respect to a position where the magnet roll comes most closely to the image holding body, wherein the main magnetic pole is provided in such a manner that when magnetic flux density in a direction perpendicular to the peripheral face of the magnet roll is measured, a series of regions where distribution of the magnetic flux density in the tangential direction is more than 90% of a peak value occupy more than 40% of an entire width of the main magnetic pole.