Power is supplied from a DC electric car line (1) through an L-C filter (3) to a PWM inverter (4). The three-phase AC output from the inverter (4) is applied to an induction motor (5) for driving an electric car. The output frequency of the inverter (4) is determined by adding or subtracting the slip frequency to or from the speed frequency of car. On the other hand, the output voltage of the inverter (4) is controlled by PWM so that the deviation as a result of comparing the current command to motor and the actual motor current is reduced to zero. In this case, an electric oscillation of current flowing from the DC electric car line (1) to the filter (3) is detected by a CT (13) with an air gap, and the electric oscillation is suppressed by adjusting the output voltage or frequency of the inverter.