An image forming apparatus forms an image on a recording material, detachably mounts a process cartridge including a photosensitive drum, and includes first and second positioning portions respectively positioning the drum upstream and downstream with respect to a cartridge mounting direction, a coupling engaging a drum coupling and transmitting a driving force to the drum, and a support supporting the cartridge at a first position positioning the drum to the first and second positioning portions and at a second position spacing the drum away from the first and second positioning portions to permit cartridge mounting and dismounting. The support includes a guide engaging the cartridge, and a third positioning portion limiting cartridge rotation relative to the first and second positioning portions when the coupling transmits a driving force to the drum coupling and for positioning the cartridge lower portion at an upstream position with respect to the mounting direction.