To magnetize different numbers of poles with a single magnetizing device without problems.

Four magnetizing coils 3a-3d and iron cores 5 are so arranged as to sandwich a sensor magnet 6, which is for detecting the rotation of a rotor 7 supported while standing at the center of the magnetizing device, in the diameter direction of the magnet. Each core is driven by an actuator so that it is freely inserted into/pulled out from the magnetizing coil. When magnetizing four poles, all the magnetizing coils and iron cores are used for magnetization. Meanwhile, when magnetizing two poles, the iron cores corresponding to unnecessary poles are so pulled out from the magnetizing coils as to be far from a to-be-magnetized sensor magnet, so that the magnetizing magnetic field is not affected by unnecessary iron cores, and the generation of a small magnetic pole is prevented. Thereby, concerning four poles, magnetization is performed as it is, and two poles, two-pole magnetization equivalent to a two-pole magnetizing device is be performed.