PURPOSE: To prevent an overcurrent generated at the time of switching due to a decrease in a synchronizing speed of a motor during a switching time when the motor is switched to a first inverter when phases coincide by matching an output voltage phase of a second inverter operated by starting the motor to the voltage phase of the first inverter 1 which has been already operated.

CONSTITUTION: An output voltage phase of an inverter 1 is compared with that of an inverter 2, and a predicting time T of after what seconds the inverter 1 synchronizes with the inverter 2, is measured. On the other hand, a switching time T1 of a switch for switching the inverter is previously measured, set, a switching signal is output to the switch when T1=T is satisfied, and the motor is switched to the first inverter 1 side.