A color electrophotographic image forming apparatus wherein cartridges are detachably mountable to a main assembly of the image forming apparatus, includes a supporting member for supporting the cartridge, movable between an inside position and an outside position; an outer wall; an opening provided in the outer wall, the opening being passed through by the supporting member, when the supporting member moves between the inside position and the outside position; a guiding member for supporting the supporting member movably so that the supporting member moves linearly between the inside position and the outside position, wherein when the supporting member is in the outside position, the guiding member supports the supporting member, so that a downstream side thereof is pivotable downwardly about an upstream side of the supporting member with respect to a movement direction to the outside position from the inside position, wherein in the state that the downstream side is pivoted downwardly about an upstream side of the supporting member with respect to a movement direction to the outside position from the inside position, the cartridge is mounted to the supporting member from above the supporting member, and the cartridge supported on the supporting member is dismounted upwardly of the supporting member.