A method for dismounting a coupler from an electrophotographic photosensitive drum flange whose drum is usable with a cartridge detachably mountable to a main assembly of an electrophotographic image forming apparatus. The coupler receives a force rotating the drum from the main assembly when the cartridge is mounted and includes force receiving and spherical portions. The method includes the steps of inclining the coupler relative to a flange rotational axis, pushing a pin from one end to the other end thereof, both of which project from the spherical portion when the inclining step inclines the coupler, making part of the pin ride on a surface of a regulator inside the flange, and dismounting the coupler from the flange by applying a force to a free end of the coupler toward the part of the pin riding on the regulator surface.