An image forming apparatus for forming an image on a sheet includes a transfer member for transferring a developed image formed on an electrophotographic photosensitive drum onto the sheet; a cartridge mounting portion for demountably mounting a cartridge accommodating the developer; an image forming unit movable, relative to a main assembly of the apparatus in a state that the cartridge is mounted to the mounting portion, between an image forming position for effecting image formation and a mounting and demounting position, away from the image formation position, for mounting and demounting the cartridge to the mounting portion; a cleaning member for removing a developer remaining on a surface of the drum; a residual developer container, demountably mounted to the image forming unit, for accommodating the developer removed by the cleaning member; wherein the residual developer container and the cartridge are detachably mountable individually relative to the image forming unit when the image forming unit is placed in the mounting and demounting position.