A developer supply container detachably mountable to a main assembly of an image forming apparatus includes a developer discharge opening to be disposed opposite to a developer receiving opening of the apparatus main assembly; an operation member capable of performing a moving operation in a mounted state of the container; an open/close driving force transmitting portion provided to the operation member; a driving force transmitting portion for transmitting a driving force, to a apparatus shutter member, from the open/close driving force transmitting portion through an open/close driving force relay gear train provided to the apparatus main assembly; and a container-side abutting portion for abutting against a gear-side abutting portion provided to at least one gear of the relay gear train. When the developer supply container is mounted into the apparatus main assembly, the container-side abutting portion abuts against the gear-side abutting portion to rotate the relay gear train toward an initial position.