The present invention has an object to detect a three-phase AC motor current with high accuracy only with DC input current measuring means to a three-phase PWM inverter, and reduce resonance of a DC power supply by a ripple current in driving an AC motor with a DC current via a three-phase PWM inverter. A carrier wave Cry_e earlier than that of a V phase is used to sample a U phase and a carrier wave Cry_l always later than that of the V phase is used to sample a W phase. Thus, a sampling time sufficiently longer than an interval Ts of the carrier waves Cry_e, Cry_m and Cry_l can be ensured, and the current can be easily detected.