A multi-phase AC motor driving device in which occurrence of failure is not erroneously determined is provided. In a multi-phase AC motor driving device including an inverter circuit; current detecting resistances Ru, Rv, and Rw, respectively arranged on lower arm of the respective phase of the inverter circuit, for detecting phase current of the motor; and a control portion and a PWM circuit for controlling ON/OFF operation of switching devices of the inverter circuit, the determination on the occurrence of failure based on the current values detected by the current detecting resistances is not made if relays connected between the inverter circuit and the motor are turned ON and all the switching devices of the lower arms of the respective phases are turned OFF.