PURPOSE: To enable to stably control an electric railcar without producing a vibrating current of a main circuit by setting a slip frequency to the prescribed minimum value except zero and the modulation factor to zero or the prescribed minimum value near zero at the idle operation controlling time.

CONSTITUTION: A current command value calculator 23 outputs a current command value IC in response to a movement command MC, a variable load signal VL and a power drive command P or a brake command B, and a reference slip frequency calculator 25 outputs a reference slip frequency fSO in response to the current command value Ic through the minimum value limiter 27. A modulation factor calculator 31 calculates a filter capacitor voltage EC and a modulation factor (γ) from an inverter frequency (f). A modulation factor selector 41 outputs normally the modulation factor (γ) and the minimum modulation factor min at the slip operation time through a modulation factor variable limiter 43. A PWM modulator 32 controls the inverter in accordance with the frequency (f) and the modulation factor (γ).