To prolong a span of service life in each individual cylinder of an internal combustion engine, making improvements in air supply and exhaust efficiency, and also to form the engine into being compact in size.

A tetragon is used in a cylinder liner (a) of an internal combustion engine, installing each of small radii (b) at the four corners, and a piston 1 uses the same type as well. In time of a crank motion, abrasion becomes increased in the crank moving direction and its counter direction of a cylinder inner wall surface by motion of the piston 1. Load is dispersed at each of longitudinal one-side surfaces of this tetragonal piston 1, reducing the abrasion, widening each tension of pressing pieces, and further, each width of airtight pieces 2 and 3 is widened as well, whereby it is made pressable up to a place being large in the degree of wear and thus engine service life may be kept longer. The form of a cylinder head is nearly turned to a tetragonal roof type. With this, when supply and exhaust valves are installed therein as much as four in total, each individual valve surface is installable wider than ever before. In addition, since a tubular form of the cylinder liner (a) is of a square pole, a flow of air supply and exhaust is formed into a continuous current manner so that air supply and exhaust efficiency is well improvable. In the case where those of cylinders in the same piston displacement each are arranged in lines and compared with the conventional ones, and engine of this cylinder liner (a) is manufactured to be compact in size.