A control unit distributes a motor voltage vector corresponding to an output request for a motor to a first and a second inverter voltage vectors associated with outputs from a first inverter and a second inverter, and determines whether a switching condition for three-phase-on mode is satisfied. Determining that the switching condition is satisfied, the control unit switches to three-phase-on mode in which every high-side switching element or every low-side switching; element of one inverter is turned on and one end of a coil in each phase of the motor is brought into common connection, and the control unit drives the motor with an output from the other inverter. Herein, the switching condition for three-phase-on mode includes failure of one inverter and an inverter voltage vector of an output from one inverter being approximate to 0 when neither of the inverters fails.