The image forming apparatus has an electrophotographic photosensitive member and a developing device for using a developer including toner and carrier, wherein the developing device develops an electrostatic image on a photosensitive member by putting the developer forming a magnetic brush on a developer bearing member including magnetic field generation device inside in contact with the photosensitive member, and when a contact pressure of the developer on the developer bearing member against the photosensitive member is P (Pa), a circumferential velocity of the developer bearing member is V.sub.Sl (mm/s), a circumferential velocity of the photosensitive member is V.sub.Dr (mm/s), and an elastic deformation ratio of the photosensitive member is W (%), a degree of sliding of the photosensitive member by the developer represented ranges from 650 to 60500.