A color electrophotographic image forming apparatus includes a plurality of electrophotographic photosensitive drums configured to develop images, an intermediary transfer member onto which developed images are transferred, and an image forming unit having a plurality of cartridge mounting portions for demountably mounting cartridges. The image forming unit is movable between a transfer position for transferring, onto the recording material, the developed images transferred onto the intermediary transfer member, and a mounting and demounting position for mounting and demounting the cartridges relative to the cartridge mounting portions. An opening permits mounting and demounting of the cartridges relative to the cartridge mounting portions of the image forming unit placed in the mounting and demounting position, and a locking member releasably locks the image forming unit which is moved to the mounting and demounting position.