To solve the problem that a counter-electromotive force is generated in a motor winding of a general-purpose motor by a slip of a motor and a backlash of a drive unit (gear) even if output frequencies of each inverter of a plurality of general-purpose motors are kept the same.

The inverters 8a and 8b to control 2 pieces of general-purpose motors 5a and 5b have converters 14a and 14b, capacitors 15a and 15b to smooth output voltages from the converters 14a and 14b, output bridge portions 16a and 16b to convert direct-current voltages smoothed by the capacitors 15a and 15b into three-phase alternating currents, inrush-current control resistances 18a and 18b connected in series between the converters 14a and 14b and the capacitors 15a and 15b, and first switches 19a and 19b connected in parallel to the inrush-current control resistances 18a and 18b. The first switches 19a and 19b are turned ON when capacitor voltages of the capacitors 15a and 15b reach a first prescribed voltage.