PURPOSE: To improve consumption of fuel and emission of exhaust even if the fuel is of low quality with a low cetane number, by using dimethyl ether as a combustion improving agent and improving combustion of the fuel.

CONSTITUTION: In case of using fuel of low quality with a relatively low cetane number, dimethyl ether as a combustion improving agent is supplied into a cylinder 1 through an intake pipe 4. That is, a reaction chamber 6 is provided in the half way of an exhaust pipe 5, and a catalyzer of platinum system or chrome oxide is filled in said chamber 6, then methyl alcohol is supplied from an inlet pipe 8 of said chamber. At starting of an engine, if the reaction chamber 6 is heated by exhaust gas flowing in the exhaust pipe 5, the dimethyl ether is produced like, for instance, 2CH3OH→CH3OCH3+H2O, and supplied into the cylinder 1 with intake air through the intake pipe 4 from an opening pipe 7. In this way, combustion of the fuel of low quality can be promoted.