PURPOSE: To improve the purifying performance of exhaust, by enabling the initial value of a control current, when an engine is transferred from an open control to a feedback control, to be changed in accordance with the time after the engine is started so that the optimum initial value can be set in accordance with each 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 in the halfway of 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 valve 2 in accordance with the actually measured air-fuel ratio for obtaining preset air-fuel ratio. Here the first and second initial value setting means 4a, 4b, setting the initial value of different feedback signal, are provided. An an engine, in which the time after its start is measured by a timer means 5, controls each setting means 4a, 4b to be switched by a switching means 6 in accordance with the measured time of said means 5.