To set an air fuel ratio and an octane number in response to the composition of vaporized gas of compressed natural gas, which is changed by internal pressure of a high pressure fuel tank.

A hydrocarbon-composition detecting sensor 19 is arranged in a fuel line 16 connecting a high pressure fuel tank 17 in which compressed natural gas is stored, to an injector 12. The composition value of the vaporized gas of the compressed natural gas supplied to the injector is detected based on the output value of the hydrocarbon-composition detecting sensor 19, and a required air fuel ratio or an octane number which corresponds to the composition value of the vaporized gas, is set. Then, the fuel injection quantity is corrected in response to the required air fuel ratio, and the ignition timing is corrected in response to the octane number. Thus, the fuel injection quantity and the injection timing which correspond to the composition of the varporized gas supplied for combustion.