A throttle opening degree TA is converted to a reference opening area SB based on a map MAPsbta representing the relationship between the throttle opening degree and the opening area in a deposit-free state. An actual opening area SA after the deposit amount is changed is obtained by subtracting an opening area change amount dDS corresponding to a change in deposit amount from the reference opening area SB. Since the opening area change amount dDS is uninfluenced by changes of the engine operational zones, the actual opening area SA is accurately determined. By converting the actual opening area SA to a control throttle opening area TAa based on the map MAPsbta, the control throttle opening degree TAa that reflects the actual opening area with improved accuracy can be detected.