PROBLEM TO BE SOLVED: To make it possible to prevent capacity increase for a PWM inverter and reduce inverter loss by the variable control of the conducting angle of a square wave inverter in response to the DC voltage of a secondary current or the fluctuation in system voltage and adjusting in such a manner that the base voltage becomes always the same as the system voltage. SOLUTION: A control section 11 inputs a DC voltage Ed and a system voltage Vs, outputs a gate pulse signal Sg of a switching element of a square wave inverter in response to their fluctuations, and performs a variable control for the conducting angle θ of a square wave inverter. And adjustment is made to make the base voltage (effective value of fundamental wave component) Vb always the same as system voltage (effective value) Vs. Then, an increase in a voltage difference Vy between added value Vx and the system voltage Vs is restricted, and the increase in capacity of PWM inverter required for its correction is also restricted in the fundamental wave component. By doing this, the capacitor of the PWN inverter can be reduced independently from DC voltage or system voltage and the inverter loss can be considerably reduced.