For example, backflow preventing means is provided, which reduces a recovery electric current occurring in the event of backflow of an electric current so as to ensure high efficiency, high reliability, etc. The backflow preventing means includes a backflow preventing element, such as a rectifying element 3a, which is connected between an alternating-current power supply 1 and a load 6 and prevents the electric current from flowing backward from the load 6 toward the alternating-current power supply 1; and commutating means 7 for performing a commutation operation for causing the electric current to flow toward a different path that is connected in parallel with the backflow preventing element, whereby a conduction loss and a loss caused by the electric current can be reduced.