Upon detection of a fault in a DC power supply based on a voltage from a voltage sensor or a temperature from a temperature sensor, a control device controls inverters to cause AC motors to output zero output torque, and generates and outputs signals STP1, STP2 to a voltage step-up converter and a DC/DC converter, respectively. The control device generates and outputs a signal SE of an L level to system relays to cut off the system relays. Thereafter, the control device generates and outputs a signal PWMDL to the voltage step-up converter to switch control of the voltage step-up converter to voltage step-down control.