A toner supply container detachably mountable to an image forming apparatus includes a container body with a discharge opening formed thereon. A sealing member, which unsealably seals the discharge opening, includes an engageable projection engageable with an engaging member provided in the image forming apparatus and a support for displaceably supporting the portion. The sealing member is moved relative to the container body away therefrom to unseal the discharge opening. The engageable projection is with the engaging member. A releasing projection, is provided in the supporting member, for receiving, from a releasing member provided in the image forming apparatus, a force for displacing the engageable projection in a direction of disengagement from the engaging member.