Even in such an arrangement in which the throttle valve (115) is operated independently of the engine control system (200), abnormal engine behavior is ensured to be detected such that a necessary fail-safe operation should be taken. The arrangement of the invention is comprised of the electronically controlled throttle system (100), engine control system (200) and engine speed monitoring unit (280), wherein the electronically controlled throttle system (100) is allowed to monitor engine behaviors. Thereby, in the case when the throttle valve (115) is operated independently of the engine control system (200), if its engine behavior becomes abnormal relative to its drive contents, the engine system senses the abnormality, and takes a fail-safe operation such as to stop operation of the throttle valve (115) and the like.