An assembling method of a cartridge detachably mountable to a main assembly of an image forming apparatus and having a developing device, said method including the steps of: preparing a container unit provided with a developing frame; mounting a toner supplying roller to the developing frame, wherein the toner supply roller is effective to supply the toner to the developing roller, and the developing roller is effective to carry the toner to a developing position; mounting a toner layer thickness regulating member to the developing frame, wherein the toner layer thickness regulating member is effective to regulate a thickness of a layer of the toner applied on the developing roller; supporting a shaft of the developing roller at one longitudinal end thereof and a shaft of the toner supplying roller at the one longitudinal end by a side cover, and mounting the side cover to the container unit, after the step of mounting the supplying roller and after the step of mounting the toner layer thickness regulating member; and supporting a shaft of the developing roller at one longitudinal end thereof and a shaft of the toner supplying roller at the one longitudinal end by a bearing unit, and mounting the bearing unit to the container unit, after the step of mounting the supplying roller and after the step of mounting the toner layer thickness regulating member.