Three-phase windings 11-13 and 21-23 of a first common-mode transformer 1 and a second common-mode transformer 2 are connected in series through connecting lines 8r-8t, respectively. The windings 11-13 are connected to an unillustrated AC power supply by connecting lines 91r-91t. The windings 21-23 are connected to a three-phase motor by connecting lines 93r-93t and through a converter and an inverter which are unillustrated. A winding 14 for common-mode voltage detection detects high-frequency leakage currents flowing through the connecting lines 91r-91t as a common-mode voltage V1, and an output voltage V2 obtained by voltage amplification by a voltage amplifier 3 is applied to a winding 24 for common-mode voltage application in such a manner that the output voltage V2 works in generally the same direction as the common-mode voltage V1, thereby canceling out the high-frequency leakage currents through the windings 21-23. Since a voltage amplification method is used, it is possible to reduce the high-frequency leakage currents with a simpler configuration as compared to a conventional current amplification method.