A color electrophotographic image forming apparatus includes cartridge mounting portions for dismountably mounting cartridges and an intermediary transfer member onto which developed images formed on electrophotographic photosensitive drums are transferred. The apparatus also includes an image forming unit comprising the cartridge mounting portions and the intermediary transfer member. The unit is movable between a transfer position for transferring, onto a sheet, the developed images transferred onto the intermediary transfer member from the drums, a cartridge mounting and dismounting position for mounting and dismounting the cartridges relative to the cartridge mounting portions, and an intermediary transfer member mounting and dismounting position for mounting and dismounting the intermediary transfer member relative to a main assembly.