The invention relates to a control method for an internal combustion engine system, capable of preventing an internal combustion engine in a state after being automatically stopped, from falling into an unrestartable state. Upon satisfaction of a given automatic stop condition, the engine 10 being running is automatically stopped (Step 11). A value of current IEBAT flowing out of a starter battery 520 is compared with a given current value IEBAT—TH, in an automatically stopped state of the engine 10 (Step S30) to diagnosis a discharging status of the starter battery 520. If a result of the diagnosis indicates that the value of current IEBAT flowing out of the starter battery 520 is greater than the given current value IEBAT—TH, it is determined that a post-attached load is detected, and the engine 10 is restarted irrespective of the satisfaction or non-satisfaction of a given restart condition.