PURPOSE: To make it unnecessary to produce an "idle" contact output of a throttle sensor, by obtaining a datum value for detecting the "idle" opening from the output of the throttle sensor at the time when judgement is made from the operation condition of an engine that it is under idling operation.

CONSTITUTION: Judgement is made at a step 103 whether alteration of a datum value for detecting the "idle" opening is allowed or not. The condition that the above alteration of the datum value is allowed, that is, judgement can be made from the operational conditions of an engine that it is under idling operation is that the engine speed or the negative pressure is lower than a predetermined value or the opening of a throttle valve is not changed. In such cases, the datum value is obtained from the output of a throttle sensor at steps 105W108. By employing such a method, it is enabled to make it unnecessary to produce an "idle" contact output of the throttle sensor.