A developer supply container detachably mountable to an image forming apparatus includes a container body for containing developer; a discharge opening, disposed at a peripheral surface of the container body, for permitting discharge of the developer therefrom; a feeding device for feeding the developer toward the discharge opening by rotation of the containers body; and a detector for detecting the amount of the developer remaining in the container body. The detector has a detection area which at least partially overlaps the discharge opening as seen in a direction perpendicular to a longitudinal direction of the developer supply container.