When an engine automatic stop judgment unit (101) judges that an engine automatic stop condition is satisfied so that a fuel injection control unit (105) stops fuel supply to an engine and an ignition control unit (106) stops ignition of the engine, and when an engine restart judgment unit (102) judges that an engine restart condition is satisfied before the engine stops, the pinion gear is driven to rotate. Then, engagement between the ring gear and the pinion gear is started when a deviation between an rpm of the engine detected by an engine rpm calculation unit (104) and an rpm of the pinion gear becomes smaller than a predetermined threshold value. Ignition by an ignition control unit (106) is inhibited during a period from start of the engagement until an engagement completion judgment unit (103) judges that the engagement is completed.