PURPOSE: To conduct a stable change-over of the operation of a motor by issuing a connection command in advance before the number of revolutions of the motor corresponds to the maximum output frequency of a variable-frequency power supply.

CONSTITUTION: When a contactor 16 is first turned OFF to stop a feed at the time of a change-over of the operation of a motor 17 by a commercial power supply 11 to that by a variable-frequency power supply 14, the number of revolutions of the motor 17 lowers gradually. The lowering of the number of revolutions is monitored, the time for the number of revolutions to become the number corresponding to the maximum output frequency of the variable- frequency power supply 14 is forecast, and the closing command of a switch 15 is issued from a change-over control circuit 19 at the time by the operating period of time of the switch 15 before the time. When the switch 15 is closed and the variable-frequency power supply 14 is connected with the motor 17, the number of revolutions of the motor 17 corresponds to the maximum output frequency of the variable-frequency power supply 14 so that the change-over is conducted smoothly.