A coil temperature estimation unit 12 estimates the coil temperature of a motor 1 from the equivalent resistance, heat capacity and thermal resistance of the coil of the motor 1, which are given in advance, from the applied voltage to the motor 1 detected by a voltage detector 17, and from a duty ratio command value input from the outside. When a temperature abnormality deciding unit 13 decides that the coil temperature is not normal, a duty limiter 14 limits the duty ratio command value at a low value and outputs it to a FET driving circuit 15.