PURPOSE: To prevent the deformation of a developing blade, soiled printing and toner leakage.

CONSTITUTION: This developing device is provided with a high-rigidity base plate 31 interposed between the developing blade 15 and the front attaching surface 12 and forming an attaching reference plane 32 equivalent to the width of the blade 15, and a pressing plate 35 attached to face to the base plate 31 through the blade 15 and pressing the blade 15 by the tightening force of a screw so that the blade 15 may be brought into contact with the plane 32 in the case of screwing the blade 15 on the front attaching surface 12 of a main body case 11. Then, a projected part projected to the inside from each end face of the toner layer regulating part 16 of the blade 15 in a widthwise direction is formed at the part of a sealing member on an upstream side in the rotating direction of the developing roller from the regulating part 16.