PURPOSE: To easily and promptly detect abnormality of a throttle opener when a throttle opening is in a predetermined value or more in the case of a condition that, opening fluctuation of a throttle valve is in a predetermined value or less, continued for a predetermined time or more.

CONSTITUTION: A throttle valve 12 for controlling an intake air amount of an internal combustion engine 11 is driven by a throttle opener 13 in such a manner that the throttle valve 12 is opened, when the engine is stopped, and fully closed when the engine is in starting and idle operation. Here an opening of the throttle valve 12 is detected by a throttle position sensor 14. The throttle opener 13 is decided abnormal by a means 15 when the opening of the throttle valve 12 is in a predetermined value or more in the case of continuing a condition that, fluctuation of the opening is in a predetermined value or less, for a predetermined time or more, being based on the initial value of the throttle opening and the opening of the throttle valve 12 detected after the engine is started.