An electrophotographic image forming apparatus includes an electrophotographic photosensitive drum and a developing cartridge. The developing cartridge is detachably mountable to a main assembly of the apparatus. A cartridge supporting member is movable, while detachably mounting the developing cartridge, between an inside position that is inside of the main assembly of the apparatus to mount the developing cartridge to the main assembly of the apparatus, and an outside position that is outside of the main assembly of the apparatus to remove the developing cartridge from the main assembly of the apparatus. When the cartridge supporting member detachably supports a plurality of such developing cartridges, and when the cartridge supporting member is moved from the outside position to the inside position, force receiving portions of the developing cartridges are contacted to force applying portions of the apparatus at different points of time.