7 In a developing device (44), a developing roller (84) is disposed at a fixed position to oppose an electrostatic latent image-forming surface (42) rotating in one direction, and a submagnet roller (102) is disposed to be parallel to the electrostatic latent image-forming surface (42), to have an axis perpendicular to the one direction, and to oppose the developed area opposing the developing roller (84) at a downstream side from the developing roller (84) along the one direction. The developing roller (84) supplies a magnetic developing agent by a magnetic force to that portion of an electrostatic latent image which opposes the developing roller (84) and is defined as a developed area to develop the electrostatic latent image in the developed area by the magnetic developing agent. The submagnet roller (102) has a plurality of segments (102a) such that different magnetic poles (N.S) thereof alternately appear along the axis of the submagnet roller (102) to remove unnecessary particles of the magnetic developing agent which are deposited on that portion of the image-forming surface (42) on which the electrostatic latent image has been developed by the developing roller (84).