A developer supply container detachably mountable to a main assembly of an image forming apparatus includes a developer containing portion; a developer discharge opening disposed at the developer containing portion; a container side engaging portion engageable with a main assembly side engaging portion; an operation member; and a transmitting portion in the operation member for transmitting a driving force to a relay portion in the main assembly for opening and closing the developer discharge opening. In a container mounted state, the operation member is capable of opening and closing a main assembly shutter member for opening and closing the developer discharge opening through the relay portion. The container side engaging portion engages with the main assembly side engaging portion when the operation member is moved in a opening direction for opening the main assembly shutter member and releases the engagement when moved in a closing direction for closing the main assembly shutter member.