The disclosed system is provided with: an engine (10) that is the motive power source of a vehicle (1); an electricity storage device (13); an assist device (12) that supports the operations of a driver by consuming power from the electricity storage device, causing the operation of a driving device (30) that alters the driving state of the vehicle; and a starting device (11) that consumes power from the electricity storage device, starting the engine. During execution of inertial driving wherein the engine is suspended and the vehicle is driven by inertia: the engine is automatically started by means of the starting device on the basis of physical quantities relating to the engine; and while the assist device is causing the operation of the driving device, starting is prohibited.