An image forming apparatus for forming an image on a recording material in a state in which a drum
cartridge including a photosensitive drum is detachably mountable to a main
assembly of the apparatus and in a state in which a developing
cartridge including a developing roller for developing with a developer an electrostatic
latent image formed on the photosensitive drum and including a developer accommodating portion for accommodating the developer is detachably mountably to the main
assembly, is constituted by a
cartridge supporting member movable, in a state of detachably supporting the drum and developing cartridges, between an inside position in which the cartridge supporting member is located inside the main
assembly and from which the drum and developing cartridges are movable to an image forming position in which the drum and developing cartridges effect
image formation and an outside position in which the cartridge supporting member is located outside the main assembly and in which the drum and developing cartridges are mountable and demountable; and a preventing member, provided to the developing cartridge, for preventing demounting of the drum cartridge from the cartridge supporting member in a state in which the developing cartridge is supported by the cartridge supporting member.