A ring gear to be coupled to a crankshaft of an engine; a starter motor for starting the engine; a pinion gear for transmitting rotation of the starter motor to the ring gear; pinion-gear meshing-engagement unit for moving the pinion gear to achieve meshing engagement with the ring gear; rpm-difference acquiring unit for acquiring an rpm difference between the pinion gear and the ring gear; and restart control unit for allowing the pinion gear to be moved by the pinion-gear meshing-engagement unit when the rpm difference between the pinion gear and the ring gear, which is acquired by the rpm-difference acquiring unit, becomes smaller than a threshold value are provided. The restart control unit includes threshold-value setting unit, and the threshold value corresponding to a characteristic to be input to the threshold-value setting unit is preset.