PURPOSE: To improve emission and operating property while the output values of sensors are low by a method wherein an average value of feedback signals upon normal operation is memorized and an air-fuel ratio is set higher than the logical air-fuel ratio by a cooling water temperature, the revolving number of the engine, an atmospheric vacuum or the like.

CONSTITUTION: The average value of control signals during a given period of time upon the feedback control under the normal operation of the engine is memorized beforehand in an ECU 19. Next, when the cold engine is started, if the cooling water temperature 20 or the output of an O2 sensor 23 is below a given value, the feedback control is switched to an open control. In this case, the air fuel ratio, obtained from the memorized average value, is outputted from the ECU 19 in a pattern characteristic having a content corresponding to the cooling water temperature 20, a load condition 21, the revolving number 22 of the engine, the output of the O2 sensor 23 or the like, and proportional control valves 12, 15 are operated by the control command thereof to set the air-fuel ratio during stopping the feedback to somewhat richer than the logical air-fuel ratio. According to this method, the emission and the operating property may be improved and absorption of the variety of carburettors upon mass production as well as the constitution of the system may be simplified.