PURPOSE: To increase the suction gas density and to improve the filling efficiency of an engine by feeding a liquidized fuel to a mixture gas cooling vaporizing device when the temperature of the mixture gas at the lower stream of a vaporizer is at a specific value or higher.

CONSTITUTION: An LP gas cylinder 1 and a vaporizer 3 are connected through a switching valve 21. At the outer wall 24a of a suction gas passage 24, a vaporizing device 23 is furnished, and the outward passage 22a of a branch passage 22 is connected to the vaporizer 3 through the switching valve 21. A temperature switch 31 is furnished at the suction gas passage 24, and when the mixture gas temperature rises to a standard value or higher, the switching valve 21 is converted to feed the liquidized fuel to the vaporizer 3 through the vaporizing device 23. In such a way, the mixture gas is cooled to increase the density, and the filling efficiency is improved.