In a magnet roll comprising a plurality of magnet pieces adhesively secured to a supporting shaft, to increase the magnetic flux density of a particular magnetic pole, the magnet pieces are disposed so that they have repelling magnetic forces in the interface between the magnetic piece having said particular magnetic pole and the magnet piece adjacent thereto. Further, in a unitarily molded magnet roll comprising a unitarily molded magnet secured to a supporting shaft, the cross-sectional shape of the shaft is selected in accordance with magnetized poles, whereby the positions of the magnetic poles are prescribed relative to the shaft and a strong magnetic force is obtained.