A color electrophotographic image forming apparatus for forming a color image on a recording material includes plural cartridge mounting portions for mounting cartridges; a single intermediary transfer member opposed to plural electrophotographic photosensitive member drums, with developed images formed on the drums to be transferred; an image forming unit including the cartridge mounting portions and the intermediary transfer member, the image forming unit being movable between a transfer position for transferring a developed image to the recording material and a mounting and demounting position for mounting and demounting the cartridges relative to the cartridge mounting portions; and a mounting and demounting position locking member for locking the image forming unit to prevent movement to the transfer position relative to a main assembly when at least one of the cartridges is not mounted in a corresponding cartridge mounting portion.