A synchronization control circuit transforms a three-phase AC voltage supplied from a commercial AC power supply to a vector on rotational coordinates and calculates a first phase difference between the vector and a d axis. When magnitude of the first phase difference is larger than a prescribed value, the synchronization control circuit sets the first phase difference as it is as a second phase difference. When magnitude of the first phase difference is smaller than the prescribed value, the synchronization control circuit generates the second phase difference in reverse polarity to the first phase difference, and controls a frequency of a clock signal so as to set the second phase difference to 0 degree.