PURPOSE: To suppress fluctuation by employing the voltage obtained by subtracting a predetermined voltage from DC output voltage as a detection voltage and controlling the output of a high frequency generating means.

CONSTITUTION: An inverter comprises a DC power source E, a high frequency generating means C, a DC output voltage detecting circuit section A, and a control circuit section B, and the inverter feeds power to a loas load D. Output voltage of the detecting circuit section A is set to a level obtained by subtracting predetermined voltage VK from DC output voltage VE, then it is fed to the control circuit section B. Since variation of power supply is detected with a high variation rate, power control can be carried out accurately.