PURPOSE: To prevent an overrich phenomenon at quick deceleration time without using an expensive constitutional element.

CONSTITUTION: An EACV5 provided in a bypass passage 4 detouring around a throttle valve 3 of a carburetor 1, first vacuum switch 17 turned off in the case of an intake pressure decreased less than the first preset pressure and the second vacuum switch 18 turned off in the case of the intake pressure decreased less than the second preset pressure set to a vacuum side from the above-mentioned first preset pressure are provided. In the case that a time from turning off the first vacuum switch 17 to turning off the second vacuum switch 18 by decelerating a vehicle is shorter than a preset value T2, the EACV is maintained fully opened only for a required time T3.