PURPOSE: To improve a fuel consumption rate in an engine adapted to burn both a liquid fuel and a modified gas, by independently predeterming and controlling a flow rate ratio and an excess air ratio of the modified gas in accordance with an engine load factor.

CONSTITUTION: Supplied to an engine 4 is an air passing through an air valve 29, a modified gas passing through a gas valve 22 and an alcohol passing through an injection valve 10. An engine load factor and consequently a gas- fuel ratio (ratio of flow rate of modified gas to a total fuel quantity) and an excess air ratio are independently computed and predetermined by means of a controller 31 adapted to receive signals outputted from a sensor 32 indicative of a magnitude of accelerator operation, a sensor 33 indicative of an engine speed and a crank angle and a sensor 28 indicative of an air flow rate and the like, thereby controlling an opening amount of each supplying valve. In this manner, an excess air ratio can be always controlled to maintain an optimum fuel consumption rate during operation at any gas-fuel ratio.