A transformer (57) using exhaust gas heat as a heat source is provided at a fuel supply system (5) of a gas engine (1), wherein the transformer (57) causes a transformational reaction (H.sub.2+CO.sub.2.fwdarw.CO+H.sub.2O) between hydrogen and carbon dioxide, thus changing the composition of the fuel, and this fuel with a changed composition is supplied to a combustion chamber. Furthermore, water vapor is produced by an exhaust heat boiler (52) using exhaust gas heat as a heat source, and this water vapor is supplied to a fuel supply system. Further still, a hydrogen separating device (56) is provided that separates and extracts hydrogen from a fuel, and the fuel is supplied to a combustion chamber after hydrogen is separated and extracted.