To effectively catch vapor and rapidly suck and deliver vapor to an intake passage by using characteristics that vapor moves round along the circumferential wall section of a fuel chamber and grows.

Two or more vapor bleeding grooves 16 extended from a periphery to a communicating opening 13 with a fuel jet 14 are arranged in the upper wall section of the fuel chamber 4 of a diaphragm type carburetor. The vapor bleeding grooves 16 communicate with the intake passage 2 through the fuel jet 14.