To set the adjust sensitivity of an air flow rate adjust needle valve in an air passage for communicating the more upstream side part than a throttle valve in an intake passage with the throttle hole of the throttle valve properly.

A columnar throttle valve 17 having a throttle hole 17b is arranged in the intake passage 44 of a carburetor main body 12. A needle installed on the throttle valve 17 is inserted in the fuel nozzle 16 projected from a constant pressure fuel room to the throttle valve 17. The air flow rate is controlled by the rotation of the throttle valve 17 and a fuel flow rate is controlled by the movement in the axial direction of the throttle valve 17. An air flow rte adjust needle valve 43 is installed in the air passage 41 for communicating the throttle hole 17b of the throttle valve 17 which the more upstream side part than the throttle valve 17 in the intake passage 44 and the air flow rate in the air passage 41 is ajusted continuously by the air flow rate adjust needle valve 43. The caliber of the more downstream side part than the throttle valve 17 in the intake passage 44 is made larger than that of the more upstream side part than the throttle valve 17 in the intake passage 44.