The control device, which is driven by an engine mounted on the vehicle, generates electric power and determines a power generation voltage for supplying the electric power to an electrical load and an electricity storage unit, comprising an electricity storage amount detection unit for detecting an amount of electricity stored in the electricity storage unit, by integrating an input/output current to/from the electricity storage unit a voltage lowering unit for lowering the power generation voltage when a predetermined electricity storage amount is detected by the electricity storage amount detection unit, a time measuring unit for measuring time elapsed after termination of charging/discharging of the electricity storage unit, and a lowering prohibition unit for prohibiting the power generation voltage from being lowered by the voltage lowering unit if the time elapsed after termination of the charging/discharging is longer than or equal to a predetermined time.