PURPOSE: To obtain a rich fuel mixture irrespective of operation of a choke valve, and improve the startability, by adjusting an opening degree of a main air bleed in response to the opening/closing of the choke valve at starting of an engine when engine temperature is not greater than a predetermined value.

CONSTITUTION: When a choke knob 21 of a manual choke opening/closing mechanism 20 is drawn to close a choke valve 3, and an engine is started to carry out warm-up operation, a choke opening/closing detecting switch 16 is turned on by the drawing of the choke knob 21. Accordingly, even when engine temperature is not greater than a predetermined value, and a temperature detecting switch 17 is in an off state, a solenoid coil of an adjusting valve 13 is excited to retract a valve member 14 of the adjusting valve 13 and make a second branch passage 25b communicated. As a result, air is supplied from the second branch passage 25b in addition to a first branch passage 25a to a main fuel passage 9. Accordingly, overrichment of a fuel mixture may be prevented, and an optimum concentration of the fuel mixture may be obtained by the choke operation.