To improve the fuel economy, high EGR combustion stability is improved by the addition of hydrogen with fast burning speed in air-fuel mixture. In order to realize on-board hydrogen gas generation, an engine reformer system assembled with a reformer catalyst and a fuel injector on the external EGR line is explored. Reformed gas is consisted with not only hydrogen but also CO, CH₄ and so on, therefore the effect of reformate gas component except for hydrogen for high EGR combustion performance is one issue of this system. This effect is clarified by the result of single cylinder engine test with simulated reformate gas and real reformate gas.