PURPOSE: To reduce the thermal influence to sensors in a carburetor and increase their detecting effect, by integrally equipping the atmospheric and intake air pressure sensors to a float chambers in the carburetor.

CONSTITUTION: Between a float chamber 16 and fuel passage 24 of a carburetor 12 bypass fuel passages 30 open-close operated by a solenoid valve 26 are provided. The solenoid valve 26 is operated by a control unit 36, that receives a signal from a detector 32 for various kinds, to adequately control an air-fuel ratio. Then an atmospheric pressure sensor 38 and intake air pressure sensor 40, used as a detector, further the control unit 36 are provided in the side wall of the float chamber 16, and the atmospheric and negative pressures sensed by the both sensors 38, 40 are introduced through passages 42, 44 similarly constructed in the surroudings of the float chamber 16.