PURPOSE: To improve the intake and exhaust efficiency of an internal combustion engine and to reduce the size and the weight of the internal combustion engine and to enhance performance by eliminating adjustment of a valve gap and eliminating a water pump and a cylinder head.

CONSTITUTION: A feed hole 8 through which fuel-air mixture is fed to the inner part 7 of a cylinder in a manner to work according to a crank angle and a hole 12 through which combustion gas is exhausted from the inner part 7 of the cylinder to the outside are formed in a cylinder 1 being of single double structure. A cylinder 4 is assembled in a manner to form a combustion chamber 16 at the inner part 7 of the cylinder and caused to perform a role of a cylinder head. Further, a gap between the outer cylinder 2 of the double structure cylinder 1 and the inner cylinder 3 thereof forms a cooling water passage 10, and a plate-form spring 36 is attached to the outer cylinder 2 of the double structure cylinder 1. Through pressurization and inflow of cooling water to the cooling water passage 10, the cooling water is drained to the water jacket of an internal combustion engine to cool the whole of the internal combustion engine.