An electric motor includes a stator having a core back which varies in width in a circumferential direction, and a rotor provided with permanent magnets. In the electric motor, cogging torque is generated M times during one revolution of the rotor, due to the number of magnetic poles of the rotor and the varied shape of the core back. The core back is provided with a step shift by an angle obtained by multiplying (360/M/2) degrees with an odd number. According to one embodiment, the core back may be provided with holes, concave portions or convex portions, so as to provide the core back with the step shift.