A gear unit for transmitting a rotational force to a rotatable member carrying developer and to which a coupling member is mountable. The gear unit includes a hole portion which opens outwardly in a direction along a rotational axis of the cylinder gear and which has an opening capable of accommodating the spherical portion, a groove portion which continues from the hole portion and which is capable of accommodating the projected portion, and a limiting portion for limiting inward movement of the spherical portion in the direction along the rotational axis of the gear. A closing member is provided for partially covering the opening of the hole portion and mounted to the gear to limit outward movement of the spherical portion in the direction along the rotational axis of the gear. The closing member is constituted by a plurality of independent pieces.