To provide an air supply device (a carburetor), dispensing with a link mechanism to achieve simplification, and preventing stop of an engine due to air-fuel mixture of dense fuel and air in idling and decelerating operation state.

This air supply device includes: a body 18; a fuel/air mixing passage 16 having a Venturi area 22 disposed between an air supply port area and a mixture outlet area 28 and regulated by the body 18; and a closing position, an idling position and a fully opening position for starting the engine. Further, the air supply device includes: a throttle/choke valve 12 disposed in the air supply port area 14 of the air/fuel mixing passage 16; a fuel metering device 34 for letting fuel flow into the Venturi area 22 through a fuel supply passage 30; and a reference passage 52 communicating the fuel metering device 34 with the air supply port area 14 on the downstream from the throttle/choke valve 12 in the idling position. In the air supply device, when the throttle/choke valve 12 is located in the idling position, in order to limit a fuel flow through the fuel supply passage 30, negative pressure is supplied to the fuel metering device 34 through the reference passage 52.