A coupling member of a toner supply container in a toner supply system detachably mountable to an image forming apparatus, includes a rotatable engagement portion provided with a hook portion which is hook-engageable with an engagement member provided in the image forming apparatus to receive a rotational force for discharging the developer in the container; a release portion for receiving a force, for displacing the hook portion inwardly in a radial direction, from a hollow release member provided in the image forming apparatus so as to release engagement of the hook portion with the engagement member; and a regulation portion provided with a hollow cylindrical portion in which a regulation shaft provided in the image forming apparatus is insertable to regulate an inclination of the coupling member by contact with the regulation shaft when the coupling member is moved away relative to the engagement member in an axial direction of the engagement portion in a state in which the release portion receives the force from the hollow release member.