An image forming apparatus includes a detaching and pressing mechanism for releasing developing rollers of developer cartridges mounted in a drum unit from photosensitive drums and pressing the developing rollers on the photosensitive drums. The detaching and pressing mechanism includes a pair of translation cam members, intermediate members provided on the translation cam members, cam holders for holding the translation cam members in such a manner as to be linearly movable in an anteroposterior direction, and a synchronous movement mechanism for allowing the pair of translation cam members to linearly move in synchroniztion.