A change-over switch performs switching between two different power supplies after connecting both power supplies simultaneously for a given time period. A controller controls the change-over switch to perform the switching operation around a synchronous point of the power supplies. Voltages of the power supplies are monitored to derive a difference voltage therebetween. From the difference voltage, a phase difference between the voltages of the power supplies is monitored. A plurality of low voltage detectors are provided for monitoring the difference voltage. The low voltage detectors are arranged to be operated at voltage values no more than preset values, respectively. The controller outputs a switching command to the change-over switch when time intervals between adjacent operation start time points of the low voltage detectors are all greater than preset values, respectively, and when the low voltage detectors are all operated. The controller controls the switching operation of the change-over switch while the monitored voltages of the power supplies are within a preset range.