PURPOSE: To improve the purifying performance of exhaust, by changing a guard valve (upper limit value of a feedback signal) so that air-fuel ratio can be controlled to the theoretical air-fuel ratio, when an engine is in a cold state continuing a signal of an air-fuel ratio sensor to be placed in a rich condition.

CONSTITUTION: An air bleed passage 1 is formed communicating with a metering jet of a carburetor adjusting a passage area by a metering needle, and an air bleed control valve 2 is provided interposing halfway said passage 1. And this air bleed control valve 2 is controlled by a feedback signal forming means 3 which forms a feedback signal controlling the opening of said control valve 2 in accordance with the actually measured air-fuel ratio for obtaining preset air-fuel ratio. Here a guard means 4, which sets the upper limit value (guard value) of the feedback signal when an engine is in a low temperature, is provided. And the guard value can be corrected by a guard value correcting means 5 in accordance with the time of duration of a rich signal in the feedback signal.