A motor driving device is operable with low current consumption at low cost and can be started up without delay. The motor driving device includes inverter circuits, relay circuits, power supply stabilizing capacitors, pre-charge circuits, and a control unit. The inverter circuits individually drive coil sets of a motor. Each relay circuit is provided between a power supply and a corresponding one of the inverter circuits. Each capacitor is provided between a corresponding one of the relay circuits and a corresponding one of the inverter circuits. The pre-charge circuits, which are provided corresponding individually to the capacitors, each charge a corresponding one of the capacitors before the relay circuits are driven. The control unit controls the pre-charge circuits so that the pre-charge circuits charge the capacitors at mutually different timings or alternately when failure diagnosis is performed on the relay circuits.