This specification discloses a developer treating device for preventing scattering or similar phenomenon of developer in an electrophotographic copying machine. The device is applicable to a developing device, a cleaning device or the like disposed in opposed relationship with the surface of a photosensitive medium to be developed or to be cleaned. In order that the flow of developer may be positively utilized to prevent the same from flowing out of the developing device or the cleaning device, the device has a developer scatter preventing member such as stay or the like which forms a multistage slit between itself and the surface of the photosensitive medium.