An image forming apparatus is provided for forming an image on a recording material. The apparatus includes a plurality of cartridges, and a cartridge supporting member including a plurality of mounting portions in which the plurality of cartridges is detachably mountable. A plurality of preventing members are each provided movably in an associated cartridge, corresponding to the plurality of mounting portions. Each preventing member is movable between a first position in which each preventing member prevents movement of the cartridge supporting member from an outside position to an inside position by contacting the main assembly, and a second position, retracted from the first position, in which each preventing member permits the movement of the cartridge supporting member.