PURPOSE: To detect both terminal voltage of a reactor by the reactor by installing a current detector to the reactor of the output circuit of an inverter, sampling reactor current and multiplying the variation section of reactor current by the inductance of the reactor in the output circuit.

CONSTITUTION: A current detector is mounted on the reactor 6 of the output circuit of an inverter, in which constant voltage operation is conducted and load 8 is supplied with AC power, reactor current is detected, and input to a both-terminal voltage detector 13, the variation section of a sampling current value for a preset sampling period is acquired, and the variation section is multiplied by the inductance L of the reactor 6, and a value obtained by multiplication is converted into both terminal voltage and output as a voltage signal. The sampling period is selected properly in conformity with the mode of load fluctuation, and the voltage loss of the reactor 6 resulting from load fluctuation is computed instantaneously, and fed forward to a voltage control system.