An assembling method for a process cartridge the assembling method includes a step of preparing a drum unit supporting a electrophotographic photosensitive drum; a step of preparing a developing unit supporting a developing roller for developing a electrostatic latent image formed on the electrophotographic photosensitive drum a developer supplying roller for supplying a developer to the developing roller and a developer accommodating portion having a developer supply opening; a unit coupling process of coupling the drum unit and the developing unit with each other; a first-end developing-device spacing member mounting step after the unit coupling process of engaging with a opening provided at a first end portion of the drum unit a first-end developing-device spacing member provided rotatably about a shaft at a first end portion of the developing unit to regulate rotation of the developing unit relative to the drum unit; and a second-end developing-device spacing member mounting step of engaging, with a projection provided at a second end portion of the drum unit, a second-end developing-device spacing member provided rotatably about a shaft at a second end portion of the developing unit to regulate rotation of the developing unit relative to the drum unit.