To improve efficiency at low load and reduce discharge amount of crocking and NOx at high load by directly injecting fuel into a cylinder just before ignition time at the low load, and injecting the fuel into an air feeding process and directly injecting the fuel into the cylinder also just before ignition timing at the high load.

When a gas injection piston 7 is on the top of a gas chamber 6, small amount of low pressure gas is supplied from a valve 18 to the gas chamber 6 with low pressure, and during high power large amount of low pressure gas is supplied from the valve 18 to the gas chamber 6 with low pressure. When a piston 4 rises in a compression process after supply of the low pressure gas into the gas chamber 6, air in a combustion chamber 21 is compressed, and goes into the gas chamber 6 from an injection hole 8 to mix with gas. When bush rod 16 is pressed up just before ignition time, a rocker arm 14 rotates counterclockwise to drop the gas injection piston 7, ignition is performed when gas 9 forms adequate mixture layered near an ignition plug 5, and subsequently explosion combustion proceeds.