PURPOSE: To ensure overcurrent protection, by detecting a current of a smoothing circuit, and in the period when the current value is larger than a predetermined reference level by stopping the operation of a second switching element.

CONSTITUTION: A voltage controlling section of a DC chopper control inverter comprises first and second transistors(Tr) 2 and 3 for performing a chopper operation of a DC power source 1, first and second diodes 4 and 5 connected in parallel to the transistors 2 and 3 respectively, a smoothing choke coil 6 and capacitor 7, and an inverter 9. The voltage controlling section feeds a current to a load, a three-phase induction motor 10. Further, provided are current detecting means 15 of the choke coil 6, a reference current value 16 with a predetermined hysteresis characteristic, current value comparing means 16 and a base signal interlocking circuit 17 for performing cutoff of the base of the second transistor 3. Thus, in the period when the current value of the smoothing circuit is more than the reference current value 16, the operation of the second transistor 3 is stopped and regenerated to the DC power source 1.