To achieve a method for effectively reforming liquid fuel by applying electromagnetic wave treatment technology to the liquid fuel.

Alternating current having a single frequency or temporally changed frequency is made to flow in coil parts 2a, 2b wound from a reduction (-) type electromagnetic wave generator 3 around outer peripheries of liquid fuel piping 1a and air piping 1b for combustion of the liquid fuel, respectively, so that the zeta potential of the liquid fuel becomes negative, so as to irradiate A.C. electromagnetic waves to the liquid fuel. Thus, by enhancing firing quality of the fuel by the A.C. electromagnetic wave treatment of the liquid fuel compared to conventional cases, fuel consumption is reduced and the concentrations of carbon monoxide, hydrocarbon and nitrogen monoxide in exhaust gas are reduced. A honeycomb-shaped wire mesh 12 is arranged in the upper part of an air filter 11 within an air cleaner 10 in the combustion air supply piping 1b. When the electromagnetic wave treatment is applied to combustion air via the wire mesh 12, torque of a vehicle is increased.