PURPOSE: To automatically perform starting through gasoline, in a carburetor used with gasoline at starting and alcohol after starting, by operating a plunger valve with a thermowax, opening a negative pressure valve through said plunger valve at the time of low temperature and opening a gasoline passage.

CONSTITUTION: A pressing element 4 is moved sliding by expansible action of a thermowax 6 connecting to a heat pipe 7 communicated with one end to the high temperture part of an engine, and a plunger valve 2 in this carburetor is adapted with its upper end to a diagonal surface 5 of the pressing element 4, then said pressing element 4 is urged with the plunger valve 2 in the direction of opening an air passage 1 by a spring 3. While the lower end of a fuel passage 8 communicated to the air passage 1 is connected to a main float chamber 10, stored with alcohol main fuel, through a jet 9 and to a gasoline passage 12, having a negative pressure valve 15, through a jet 11. When the engine is at low temperature, the plunger valve 2 is placed to the uppermost position, and a transverse hole 13 of said valve 2 opens a negative pressure passage 14 to open a negative pressure valve 15, and gasoline in a tank 18 can be introduced to the fuel passage 8 from the passage 12.