An image forming apparatus includes a cartridge including a photosensitive drum; an endless belt configured to transfer the toner image formed on the drum; an endless belt supporting member movable between a contacting position for contacting the belt to the drum and a non-contacting position retracted from the contacting position away from the drum; and a cartridge supporting member movable between an image forming position in which the drum contacts the belt taking the contacting position and a non-image-forming position in which the cartridge is movable from a inside to a outside of the main assembly. By movement of the belt from the contacting position to the non-contacting position, a space capable of accommodating at least a part of the cartridge supporting member taking the non-image-forming position and/or the cartridge supported by the cartridge supporting member taking the non-image-forming position is provided.