A blade is pressed against a developing roller for regulating the amount of toner which is deposited on a surface of the developing roller. The blade is formed with a slant face at its tip end, to which the toner is transported along a direction of rotation of the developing roller. An angle of aperture .theta. at a toner inlet portion defined by the developing roller and the slant face is set at 12.5.degree. or more, the angle of aperture counting in an angle .psi. of the slant face, elastic deformation and radius (R) of the developing roller, and an angle .alpha. related to a nip width w on which the blade contacts with the developing roller. Such an angle of aperture .theta. provides a great allowance for a free length l of the blade, thereby permitting the blade to maintain a constant toner mass for forming a uniform toner layer.