PURPOSE: To prevent incomplete combustion in a car engine using mixed fuel of gasoline and alcohol by increasing the ratio of alcohol in the fuel in accordance with the lowering of atmospheric pressure for restraining the enrichment of air-fuel ratio at the time of highland running or the like.

CONSTITUTION: During the running on a usual flatland, a bellows 11a of an atmospheric pressure detecting device 11 is almost balanced in inner and outer pressures, and an opening part 7a of a valve 9 is closed to supply gasoline only to a suction pipe. when the altitude of a running land is increased to lower the atmospheric pressure, the bellows 11a is extended by the difference between inner and outer pressures to open the opening 7a, and mixed fuel of gasoline and alcohol is supplied to the suction pipe. When the altitude is further increased, the extension of the bellows 11a is increased to close an opening part 3a, and only alcohol fuel is supplied to the suction pipe.