PURPOSE: To prevent over thickening of an air-fuel ratio or run-on when a secondary-side throttle valve of a two-stage, two-barrel carburetor has caused a sealing fault, by shutting off the secondary-system fuel, or supplying air.

CONSTITUTION: A primary-side suction air passage 25 is provided with a primary- side throttle valve 24, and a secondary-side suction air passage 34 is provided with a secondary-side slow port 40 and a secondary-side throttle valve 32. The throttle valves 24 and 32 are mechanically interlocked with a cam 70. A fuel passage 52 communicated with the slow port 40 is provided with a solenoid valve 82, to which primary and secondary-side throttle valve opening detection means 71 and 77 are connected in series. Only when both of the detection means 71 and 77 are on, the valve 82 is opened to supply the fuel from the port 40. Moreover, a secondary-air control valve 84 is arranged in parallel with both of the detection means 71 and 77, in order to supply the secondary air for an exhaust system (not shown in the figure) with either one of the detection means is on.