A control device controls a switching operation of a switching element such that an AC voltage allowing an AC motor to output torque in accordance with a torque command value is applied to a coil winding of each phase. The control device sets a gate resistance variably between a first pulse voltage at the time of reversal of polarity of the AC voltage and a remaining pulse voltage, in a drive circuit for turning on/off the switching element in response to a switching control signal. By making longer a time period for the first pulse voltage at the time of reversal of polarity to rise, occurrence of partial discharge in a gap between coil windings of respective phases is suppressed.