A magnet roller includes a magnet portion for generating magnetic force such that a plurality of peaks are formed in a magnetic force distribution. The magnet portion includes a plurality of magnetic poles. A cross section of the magnet portion includes a portion in which a distance from an axial center of the magnet portion to an outer edge of the magnet portion continuously changes in the circumferential direction. The magnetic force on a surface of a developing sleeve at a position corresponding to each of the plurality of magnetic poles is determined by (i) a distance from each of the plurality of magnetic poles to the developing sleeve in a radial direction of the developing sleeve and (ii) a magnitude of magnetic force in each of the plurality of magnetic poles. The distances from the plurality of magnetic poles to the developing sleeve in the radial direction are different.