An image forming apparatus in a state that a cartridge is mounted to a mounting portion of a main assembly of the apparatus, includes a first opening, provided in the main assembly, for permitting the cartridge to pass into the main assembly to mount the cartridge; an door provided in the main assembly and movable between a closing position and an open position; a second opening, provided in the main assembly, for permitting the recording material to pass into the main assembly to supply the recording material; a light blocking member provided in the main assembly to suppress impingement of at least a part of external light having entered through the second opening onto an photosensitive drum, wherein the light blocking member functions to guide the cartridge toward the mounting portion through the first opening which is open by movement of the door to the open position.