PURPOSE: To secure a good operation performance of an engine and perform purification of an exhaust gas sufficiently by a method wherein an air-fuel ratio of a mixture gas set by a choke valve is corrected during the time from starting in a cold state of the engine till it becomes warm in accordance with a heater temperature of a bimetal.

CONSTITUTION: A signal electric current to a control circuit 9 becomes large since a resistance of a heater 12 is small when the PTC heater 12 of an automatic choke 11 is at a low temperature. Then, the control circuit 9 outputs ON-OFf signals which make the closed time proportion large in the open and closed time ratio of an electromagnetic valve 10 and the mixture gas thin in the air-fuel ratio, until an O2 sensor 8 reaches a constant operation temperature. With this system, the electromagnetic valve 10 working as an actuator, which is installed in a bypass fuel passage 17 of a carburettor 5, is controlled and the air-fuel ratio of the mixture gas is controlled during no operation of the O2 sensor 8 to enable to prevent poisonous contents of an exhaust gas from being produced massively.