A developer supply cartridge supplies developer into a process cartridge and is detachably mountable to a main assembly of an electrophotographic image forming apparatus. The process cartridge includes a drum, a developing roller, a receiving-side shutter opening and closing a developer receiving opening receiving the developer used by the roller, a receiving-side mover to move the shutter including an operator, and a regulator regulating movement of the mover when the operator is at an operating position. The developer supply cartridge includes a developer accommodator, developer supply opening supplying developer from the accommodator into the process cartridge through the developer receiving opening, a supplying-side shutter opening open and closing the developer supply opening, and a supplying-side mover to move the supplying-side shutter from a closed to an open position when the developer supply cartridge enters and the process cartridge is set in the main assembly, by engagement with the operator.