PURPOSE: To obtain a signal similar to a square wave or a peaked wave by arranging a magnet so as to be minimized in the interval between magnetic poles while the width of the magnetic poles is made relatively large.

CONSTITUTION: Sections 10a, 10b, 10c... radially divided of a cylinder 10 are so arranged that the S pole and the N pole are alternately magnetized and adjacent poles are brought closer to each other in the interval therebetween as much as possible while the magnetic poles themselves has a relatively constant expanse. In this magnetic flux pattern, there is a growing gradient Δy/Δx in the rise to the peak from the zero point thereby minimizing the errors Δx' even when the pattern deviates by Δy due to a poor magnetization accuracy or the like. To make a magnet, a plurality of relatively broad yokes 12 are arranged in the perimeter of a cylinder 11 made of a magnetic material are arranged with the formation of a groove 13 having a clearance as narrow as possible. The coil 14 is to be used for magnetization.