A motor driving device is provided. The motor driving device includes a driving unit configured to drive a starter motor/generator that is directly connected to an engine by a rotation shaft, by performing on/off control of switching elements in a bridge circuit for driving a motor; and a driving controller configured to control a driving operation for the starter motor/generator that is performed by the driving unit. The driving unit controls a motor driving voltage for the starter motor/generator by changing ON periods of the switching elements of the bridge circuit, and turns on predetermined switching elements of the bridge circuit at a predetermined timing to perform an active freewheel operation for boosting a regenerative voltage generated by the starter motor/generator. The driving controller instructs the driving unit to control the motor driving voltage and on/off of the active freewheel operation, based on a battery voltage.