An image forming apparatus for forming an image on a recording material includes a plurality of cartridges each including a developer accommodating portion for accommodating a developer for developing an electrostatic latent image formed on a photosensitive member. The apparatus also includes a cartridge supporting member including a plurality of mounting portions in which the plurality of cartridges is detachably mountable, with the cartridge supporting member being movable between an inside position inside of a main assembly of the image forming apparatus and an outside position outside of the main assembly. An associated preventing member is positioned at a first position when an associated cartridge is incorrectly mounted in an associated mounting portion, and the associated preventing member is retracted into a second position in contact with the cartridge supporting member when the associated cartridge is mounted in the associated mounting portion.