PURPOSE: To prevent the destruction of a switching element caused by a response delay of an insulating amplifier and a malfunction of an over-current detection circuit caused by noise by taking an over-current detection signal into a driving signal cutoff circuit after insulating by a photo coupler.

CONSTITUTION: Terminal voltage of each of shunt resistances SH1-SH3 is input into an insulating amplifier 1 installed for each phase at the same time with being input into an over-current detection circuit 2. A current of each phase is compared with a preliminarily set overcurrent level. When the current of each phase is at the set overcurrent level or above, it is put into an OR circuit 10 through photo couplers PC3 and PC4. Then the output from the OR circuit is input into a base cutoff circuit 8 as a base cutoff signal S1. A base signal of a power transistor is cut off through a base driver BD1 and photo couplers PC2 and BD2.