PURPOSE:To prevent a vibration phenomenon at a commutating time by discharging charge stored in a commutation condenser in the former half of the step of commutating an AC side phase current of a current type inverter in the latter half of the step of commutating. CONSTITUTION:When a GTO3 is turned ON at the commutating time and a GTO4 is turned OFF, commutation condensers 9, 10 are discharged via Iv, thereby reducing the commutating voltage. When the current Iu becomes substantially zero, the commutating voltage also becomes zero. The fact that the current Iu becomes a substantially zero is detected by a current zero detector to turn OFF the GTO3 and ON the GTO4. Since the commutation is completed in the state that the commutating voltage is substantially zero in this manner, a vibration phenomenon after the commutation is completed cannot be almost generated.