PURPOSE: To prevent the inflow of condensed liquid into the engine by a method wherein the device is provided with a trap, reserving the condensed liquid of the modified gas, and a means to recirculate the condensed liquid to a modifier when the level of the condensed liquid, reserved in the trap, has become higher than a given level.

CONSTITUTION: The modified gas, obtained by modifying a liquid fuel by the modifier 1, is introduced into a gas cooler 2 through a pipeline 5 and is cooled, thereafter, is separated into a gas and the condensed liquid by the trap 20 and the gas is injected into a suction air pipe from a gas control valve 3 through a surge tank 41 and a gas stop valve 42. On the other hand, the condensed liquid is reserved in the vessel of the trap 20 and when the amount of the reserved liquid has arrived at a predetermined level, a condensed liquid pump 30 is energized and the condensed liquid is discharged forcibly into a liquid fuel pipeline 4B through a recirculating path 40. Thereafter, the condensed liquid is recirculated to the modifier 1 together with the liquid fuel flowing through the pipe- line 4B. The liquid fuel is supplied to the modifier 1 by a pump 10 through a regulator 43 and the like.