A control device sets reverse rotation period as a cranking prohibition period, and when a start request of an engine is generated during reverse rotation period, cranking is started by driving either or both of the first starter and the second starter when it is detected that a crankshaft is shifted from reverse rotation to forward rotation based on an information of a rotation angle sensor. As a result, the delay from generation of the start request of the engine until cranking is started can be reduced, so that the starting time of the engine can be shortened.