PURPOSE: To reduce circulation current flowing from a machine frame to the earth by connecting one ends of capacitors with the output sides of respective zero-phase reactors arranged in series between an AC source line and the input terminal of an inverter, and connecting the other ends of the capacitors together with the machine frame.

CONSTITUTION: Zero-phase reactors 7a-7c are connected in series for each phase between an AC power source 1 and the converter section 3 of an inverter 2. One ends of capacitors 8a-8c are connected with the output sides of the reactors 7a-7c while the other ends thereof are connected together with the frame 62 of a motor 6 through a connecting wire 9. Consequently, circulation current flowing through the machine frame, the earth, the AC power source 1 and the inverter 2 is reduced and the size of earth leakage breaker for the AC power source is reduced resulting in prevention of electric shock due to faulty grounding.