A premixed developing system developing device, process cartridge, and image forming apparatus, in which a developing agent thrown up by a transport member is not discharged from a discharge aperture, variation in the quantity of the developing agent discharged to the outside does not occur, and the image quality of images output is stable. A discharge aperture is formed in the wall of a transport path of a transport member to discharge the developing agent to the outside when the surface of the developing agent exceeds a predetermined level. The transport member is provided with an area, in a part in the longitudinal direction, in which a screw portion is not formed. The discharge aperture is disposed opposite to the area, where the screw portion is not formed on the transport member, and is formed such that the length of the discharge aperture is smaller than the length of the area in the longitudinal direction.