PURPOSE: To restrict vaporization of fuel when an engine is stopped, and to promote vaporization of fuel when an engine is rotated at high speed in order to supply the fuel to a carburetor by connecting particularly first and second valve chambers to the carburetor through a negative pressure introduction route.

CONSTITUTION: A first valve chamber 15 is formed by a first diaphragm 14 besides a charge route 8, in the middle of the charge route 8. A first valve element 17 energized in the direction where the charge route 8 is closed is provided with a first opening/closing valve 12 held with the first diaphragm 14. A second valve chamber 20 is formed by a second diaphragm 19 besides an air introduction route 10, in the middle of the air introduction route 10. A second opening/closing valve 13 is provided, on which a second valve element 22 for reducing the opening of the air introduction route 10 when the second valve chamber 20 becomes of negative pressure and when the second diaphragm 19 is moved to the side of the second valve chamber 20. The first valve chamber 15 and the second valve chamber 20 are connected to a carburetor 3 through a negative pressure introduction route 23.