A developing roller includes a developing sleeve having a nonmagnetic material, and a magnet roll provided inside the developing sleeve and formed by dispersing a magnetic powder in a polymer compound. A portion of the magnet roll corresponding to a developing pole of the magnet roll is equipped with a main-pole molded magnet whose magnetic force per unit of volume is greater than that of the magnet roll. A magnetic pole adjacent to the developing pole of the magnet roll downstream in the developer conveyance direction has a peak magnetic flux density on the developing sleeve greater than that of the developing pole, and has a half value width, which is the width of the magnetic pole at which a magnetic flux density of one-half the peak magnetic flux density is exhibited, greater than that of the developing pole.