PURPOSE: To improve air-fuel ratio during a certain period of time in acceleration by equipping on one end of a fuel control valve with a vapor control valve thereby injecting suitable amount of oxygen containing vapor, synchronized with fuel injection, into a cylinder.

CONSTITUTION: Peroxide solution 163 passes through a pump 162, a pre-heater 164, an electric heater 164, and catalyst 165 to become oxygen-containing vapor and reach a vapor on-off valve 200. When an accelerator pedal 210 is depressed, the vapor on-off valve 200 is opened to let the vapor into a vapor injection pipe 160. At a position on the axial line of a fuel nozzle 152 opens a vapor nozzle and on one end of a fuel valve needle 154 is provided a vapor control valve to be opened to jet mixture of fuel and high temperature vapor at a certain mixture ratio. As a result, oxygen in proportion to increased fuel during acceleration is supercharged leading to improve air-fuel ratio.