PURPOSE: To prevent the overheating of the modifier effectively by a method wherein an exhaust gas bypass valve, interposed in a modified gas path bypassing the modifier, is controlled based on the detection of the values of the temperature and flow amount of heating gas or the same corresponding to those in addition to the temperature of the modifier.

CONSTITUTION: Upon a normal operation, a liquid fuel, such as methanol or the like, is modified into a combustible gas by a modifier 35 interposed in an exhaust gas path 34, thereafter, is injected into a suction air path 40 through a gas cooler 37, a gas stop valve 38 and a gas valve 39. In such a device, an exhaust bypass valve 47 is interposed in the bypass path of an exhaust gas path 34, which is bypassing the modifier 35. The valve 47 is controlled by an actuator 48 controlled by a control unit 56. The bypass valve 47 is fully opened when the output of a modifying catalyst temperature sensor 45 has exceeded a predetermined value while the opening degree of said valve 47 is controlled in accordance with the operating condition at that time based on a modified gas flow amount limit value obtained by the output of the temperature and pressure sensors 53, 54 of the modified gas when the output of the sensor 54 is lower than the predetermined value.