PURPOSE: To reduce the change of air-fuel ratio by a method wherein vacuum in a suction air pipe is acted by a solenoid valve selectively on two kinds of actuators to correct in multiple steps the opening degree of a choke valve as the warm-up of an engine proceeds and, subsequently, correct it proportionally.

CONSTITUTION: In case a starter is driven under a low temperature, the vacuum in the suction air pipe is not being generated, therefore, respective actuators 8, 9 for correcting the perfect explosion of first step as well as for correcting the perfect explosion of the first and second steps do not operate and the choke valve 3 is closed totally by a bimetal 5. On the other hand, when the perfect explosion is effected, the vacuum in the suction air pipe is introduced into a path 24, however, the solenoid valve 26 is being closed and the vacuum in the suction air pipe acts on the vacuum chamber 13 of the actuator 8 only, therefore, the choke valve 3 is opened slightly. When the warm-up proceeds and the temperature of cooling water has become a set temperature, the solenoid valve 26 is opened and the vacuum in the suction air pipe acts also on the vacuum chamber 19 of the actuator 9, therefore, the choke valve 3 is opened step wise. Thereafter, the opening degree of the choke valve 3 is increased in proportional manner.