PURPOSE: To prevent the detecting error owing to the chattering of an idle switch, and to improve the detection response property, by continuing the ON/ OFF judgement of the idle switch for a specific period, and judging when there is no output variation.

CONSTITUTION: A contact type idle switch 6 is installed to detect the opening of a throttle valve 5, and to output the signal ON at the fully closed condition and the signal OFF at the condition not fully closed. The output signal of the idle switch 6 is input to a CPU 30 to detect the idling. Therefore, only when there is no output variation for a specific period after the output signal of the switch 6 is converted from ON to OFF (from OFF to ON), it is decided to be not fully closed (fully closed), and just after the idle switch 6 is converted from OFF to ON (from ON to OFF), it is decided to be fully closed (not fully closed) in the first judgement (the second judgement). And according to the resultant judgement, the idle rotation frequency control and the like are exercised.