An image forming apparatus for forming an image on a recording material includes a cartridge accommodating a developer for developing an electrostatic latent image formed on a photosensitive member, a supporter detachably mounting the cartridge, and a preventor. The supporter moves between an inside position where the cartridge forms an image inside a main assembly of the apparatus and an outside position where the cartridge is mounted and demounted outside the main assembly. The preventor moves between a first position in which it prevents, when the cartridge is not mounted, movement of the supporter from the outside position to the inside position by contacting a portion thereof, and a second position, retracted from the first position, in which the it permits, when the cartridge is mounted, the movement of the supporter from the outside position to the inside position.