Magnet roller, developing roller, developing device, and image forming apparatus

A magnet roller disposed inside a developing sleeve formed in a cylindrical shape, the magnet roller comprising:

• a magnet portion for generating magnetic force such that a plurality of peaks are formed in a magnetic force distribution in a circumferential direction of the magnet roller,
• the magnet portion including a plurality of magnetic poles that are arranged in the circumferential direction so as to form the plurality of peaks,
• the magnet portion having a cross section that is orthogonal to an axis direction of the magnet roller, the cross section having a shape that is formed to include a portion in which a distance from an axial center of the magnet portion to an outer edge of the magnet portion changes continuously in the circumferential direction, wherein
• magnetic force on a surface of the 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,
• distances from the plurality of magnetic poles to the developing sleeve in the radial direction of the developing sleeve are different, and
• a radius of the magnet portion between each pair of two adjacent magnetic poles in a circumferential direction is less than a radius of each of the two adjacent magnetic poles, whereby a cross section of the magnet portion has a recess between the each pair of two adjacent magnetic poles.