To remarkably improve thermal efficiency by completely preventing generation of NOx by not making the air a working fluid, preventing output decline even in the case an expansion ratio to compression is increased for improvement of the thermal efficiency, reducing pump loss and friction loss, and reducing cooling loss by lowering the maximum combustion temperature.

The four-cycle internal combustion engine comprises a poppet valve for circulation and blockage of a burned gas to a cylinder head, having steps of intake, compression, expansion and exhaustion of a working fluid. It comprises an oxygen supply container 15, an oxygen jetting valve 38 for jetting the oxygen from the oxygen supply container 15 and a fuel jetting valve 37 for jetting a fuel, and a cooling device 29 for cooling the burned gas discharged from the inside of the cylinder via the poppet valve 21. The burned gas cooled down by the cooling device 29 is introduced into the cylinder via the poppet valve 25. By changing the valve closing time of the poppet valve 25 by a variable valve gear 4 according to the operation state of the engine, the real compression ratio is changed.