A developer supply container detachably mountable to an image forming apparatus includes a rotatable container body provided with a portion-to-be-engaged on an inner surface thereof; an opening for permitting discharging of the developer; a sealing portion for sealing the opening; an extended portion being displaceable toward a rotational axis of the container body; an engaging portion engageable with the portion-to-be-engaged; and a limiting portion to substantially hold the sealing portion in a sealing position by engagement between the engaging portion and the portion-to-be-engaged, and a non-limiting portion to permit the relative movement of the sealing portion by disengagement between the engaging portion and the portion-to-be-engaged.