To suppress an induced voltage that is generated from a current flowing in field windings, when a field winding electric motor is stopped.

A motor induced-voltage estimating portion 4b of a microcomputer 4 estimates a motor induced-voltage that is generated when the electric motor 1 is stopped. A motor-stop controlling portion 4a judges whether or not the induced voltage estimated by the motor induced-voltage estimating portion 4b is equal to or lower than a given voltage. If it is larger, field-weakening control is performed by an inverter 2 to reduce the induced voltage by controlling the ON/OFF of MOSFETs 21-26 constituting the inverter 2 until the induced voltage becomes equal to or lower than the given voltage.