PURPOSE: To perform stable operation of an engine, by providing a trap storing condensed liquid in a feed line of alcohol quality improved gas and supplying the condensed liquid to the feed line of alcohol when a level of the liquid in the trap reaches not less than a prescribed amount.

CONSTITUTION: High temperature gas, improved its quality by receiving exhaust heat in accordance with operation of an engine 13, is heat exchanged in a gas cooler 2 with alcohol fed to a quality improving device 1, cooled and then fed flowing into a trap 14 while being further cooled through a gas pipe 5b. Here are seized condensed components of water, quality unimproved alcohol, ether, etc. in the quality improved gas, and only uncondensed quality improved gas is guided to a gas control valve 3 and fed to the engine 13. If quantity of the condensed liquid in the trap 14 reaches not less than a certain level, the condensed liquid in the trap 14 flows down into an alcohol pipe 6 and is injected into an intake pipe from an alcohol injection valve 7.