PURPOSE: To securely maintain output at the inital stage of acceleration by excluding the effect of dynamic pressure caused by traveling wind and constantly stablizing taking in fuel while aiming at output improvement by introducing outside air.

CONSTITUTION: The inlet port 3a of an LPG engine 1 is opened toward the front of a vehicle VH and main fuel from a regulator 13 is guided out into an intake passage 2 based on the negative pressure generated in a venturi 4. An injector 17 for injecting auxiliary fuel is also provided in the intake passage 2. An ECU 39 makes calculation of the main fuel shortage effected by the dynamic pressure caused by traveling wind, based on the results detection by an intake pressure sensor 34 and a vehicle speed sensor 38. The ECU 39 also causes auxiliary fuel to be injected by drive controlling the injector 17 on the basis of the results of the calculation. Thus the LPG engine 1 is constantly supplied with relatively low temperature outside air introduced therein. Further in case of main fuel shortage due to the effect of dynamic pressure caused by traveling wind, the shortage of main fuel is supplemented by the injection of the auxiliary fuel.