In a polyphase electric motor, a voltage is sequentially applied to a plurality of windings on a phase-by-phase basis. Then, there is sensed a rotational speed of a rotor, which is rotated by a rotating magnetic field that is generated by sequentially flowing an electric current in the windings on the phase-by-phase basis upon the sequential application of the voltage to the windings. Then, an oscillation frequency of a periodic oscillation, which is generated in the motor at the sensed rotational speed of the rotor, is obtained. Thereafter, it is determined whether the obtained oscillation frequency is a predetermined resonance frequency. Next, the voltage to be applied to the windings is corrected in a manner that reduces a resonance generated in the motor when a result of the determination indicates that the obtained oscillation frequency is the predetermined resonance frequency.