An image forming apparatus capable of easily performing exchange in a process cartridge with a new cartridge and performing removal of a jammed sheet without detachment of a drawer from a casing. The drawer detachably holds a plurality of process cartridges, and is movable in frontward/rearward direction to one of an operable position where each photosensitive drum is in direct confrontation with a conveyer belt, a cartridge exchangeable position where all of the cartridges are positioned out of the casing, and a jammed sheet processing position ahead of the cartridge exchangeable position for removing a jammed sheet. A regulation mechanism is provided for selectively regulating the movement of the drawer dependent on detachment of a rearmost process cartridge from the drawer.