PURPOSE: To increase the carrier frequency of an inverter and to detect abnormal state of the inverter accurately with high speed by providing a first current detecting means for detecting the output current from a converter, a second current detecting means for detecting the discharge current from a capacitor, and means for adding thus detected values.

CONSTITUTION: DC output current from a converter 1 is detected through a current detector 4 while the voltage across a capacitor 2 is differentiated through a differentiator 8 to produce an AC discharge current of a capacitor 2. DC output current from the capacitor 1 detected through the current detector 4 is added to the AC discharge current of the capacitor 2 in an adder 9, the sum is then compared with a detected value of DC input current to an inverter 3 and fed to a comparator 5. Since a large surge voltage is prevented, high speed switching operation can be carried out with transistors having low rated voltages.