An image forming apparatus has an abutment roller which is provided at either end of a developing sleeve in a longitudinal direction and makes an SD gap (Gsd) between a photosensitive drum and a developing sleeve constant, an abutment block which is provided at either end of the photosensitive drum in a longitudinal direction and on which the abutment roller abuts, and a pressing spring which directly presses the abutment roller toward the abutment block. The stability of the SD gap (Gsd) and reduction of the driving load torque of the developing sleeve can be achieved to cope with both high image quality and durability of the image forming apparatus.