To correctly obtain a desired fuel amount without excessively throttling a hole opening area of a main fuel jet.

A rotary throttle valve 7 having a throttle hole 7a is rotatably and elevatably fitted into a cylindrical valve chamber 3 vertically crossed with an intake passage 21 horizontally penetrated on a carburetor main body 20. A needle 5 supported to the rotary throttle valve 7 is fitted into a fuel supplying pipe 4 extending from a constant pressure fuel chamber supported to a bottom wall of the valve chest 3 to the throttle hole 7a. A second fuel jet 34a is disposed between the main fuel jet 34 of the fuel supplying pipe 4 and a fuel nozzle hole 4a.