PURPOSE: To improve the efficiency of a carburetor by evaporating fuel in a carburetor float chamber by means of an ultrasonic wave generating device.

CONSTITUTION: An evaporating chamber 7 is provided which is communicated by way of a through-hole 6 with a float chamber 1 having a float 2 contained therein. An ultrasonic vibrating plate 10 is disposed below an oil level within the vibrating chamber 7 and is caused to vibrate an ultrasonic wave generator device 12 to which a converter 13 and a battery 14 are connected. Fuel evaporated within the evaporating chamber 7 is fed through a pipe 20 and is jetted into a suction air passage 17 from a nozzle 18 for evaporated fuel which is disposed in opposition to a conventional main nozzle 16.