To eliminate an inconvenience causing an engine stall and an idle failure by inflow and accumulation of fuel by an attitude change at the time of a normal driving in an auxiliary passage for supplying air-fuel mixture for starting and outflow of fuel by the next attitude change.

In this carburetor in which an opening/closing valve 31 interlocking with a throttle valve 4 is provided in the auxiliary passage 21 connected with a downstream side of the throttle valve 4 of an intake passage 2, the opening/closing valve 31 is opened at the time of idle opening of the throttle valve 4 and replenishes air-fuel mixture to the engine and the auxiliary passage 21 is closed because the opening/closing valve 31 is closed when the throttle valve 4 is opened, a downstream side part of the opening/closing valve 31 of the auxiliary passage 21 is communicated with an upstream side part of the throttle valve 4 of the intake passage 2 by an air passage 28 and fuel is prevented from flowing into the auxiliary passage 21 by air from the air passage 28 when the opening/closing valve 31 is closed.