PURPOSE: To increase a degree of exhaust resistance in time of idling and make combustion better as well as to reduce HC content in exhaust gases, by connecting an on-off switch at a full-close position of a throttle operating body and throttling a throttle valve, while delaying ignition timing to some extent.

CONSTITUTION: When a throttle operating body 1 is operated at a full-close position, making an engine into idling, a cam 3 comes off an on-off switch 2 so that this switch 2 is connected. If so, a servomotor 6 is driven and a throttle valve 5 turns round, throttling an exhaust passage 4. Simultaneously with this, an ignition unit 7 is made to delay its ignition timing so as to cause the ignition timing to come close to zero. As a result, exhaust resistance increases, making combustion better, thus HC content in exhaust gases is reducible. On the other hand, when the throttle operating body 1 is operated in a direction increasing an engine speed, the on-off switch 2 is disconnected by the cam 3, and the throttle valve 5 opens, while the ignition unit 7 is restored to its normal ignition timing so that normal driving is performable.