Patent ID: 7761032

Claim:
An image forming apparatus of a tandem system comprising: a main body; a drum unit which is attachable to and detachable from the main body, the drum unit including: a plurality of photosensitive drums; a housing for rotatably supporting the plurality of photosensitive drums, for each photosensitive drum, a drum gear arranged at an end of the photosensitive drum for transmitting a drive force to the photosensitive drum, and for each photosensitive drum, a rotating member having a protrusion rotating with the drum gear, the protrusion being arranged at a position away from a center axis of the rotation of the photosensitive drum; a transfer unit for transferring developer color images from respective photosensitive drums onto a medium; and a guide arranged in the main body for contacting each protrusion and guiding each rotating member during attachment of the drum unit to cause each drum gear to move to a respective predetermined position, wherein the drum unit is configured so that a direction of attachment to and detachment from the main body is orthogonal to a longitudinal direction of the photosensitive drums.