PURPOSE: To actuate an overcurrent protective operation quickly upon occurrence of an overcurrent.

CONSTITUTION: When a current value detected at a current detection section 27 exceeds a protective current value command, an overcurrent limit address is generated in order to gain an access to a switching table 8. The switching table 8 contains a data for selecting a switching voltage pattern which interrupts power supply from an inverter section 3 to a three-phase induction motor 2 when an access to the switching table 8 is gained based on the overcurrent limit address. Setting of the switching voltage pattern, read out from the switching table 8, at the inverter section 3 is controlled based on the data contained in the switching table 8.