PURPOSE: To prevent afterburning from occurring, by operating a throttle opener at once at the time of supplying auxiliary fuel from the throttle opener and a coasting bypass passage at deceleration and, after delaying, supplying the auxilairy fuel compensated of atmospheric pressure.

CONSTITUTION: A throttle opener B is installed in a throttle valve 4, while a diaphragm valve A is installed in a coasting bypass passage 5. Negative pressure in a suction pipe 3 is led into the throttle opener B via vacuum passage 8a, an atmospheric pressure compensating vacuum control valve 9 and a vacuum passage 8b. A vacuum passage 8c to be branched off from the vacuum passage 8b is interconnected to a diaphragm valve A via a delay valve 10 provided with a throttle 10b and a check valve 10a. At the time of deceleration, the throttle opener B is immediately operated with the vacuum characteristic conformed to atmospheric pressure, but auxilairy fuel is supplied after being delayed in correspondence to the atmospheric pressure, and afterward it is restored to a usual state.