To attain thinning wall thickness and weight reduction of a cylinder liner by respectively forming smooth curved face parts on the outer circumferential face of the cylinder liner so that the wall thickness of the cylinder liner is gradually increased from the middle part toward upper and lower both end part sides.

A cylinder block 2 is formed with a liner bore 4 and a water jacket 6 corresponding to the liner bore 4, and a piston is provided capably of reciprocating along the cylinder bore of a cylinder liner 8 fitted into the liner bore 4. The cylinder liner 8 is formed so that the middle part is formed into fixed wall thickness, and the wall thickness is gradually increased from the middle part toward the upper end part and the lower end part. By changing the wall thickness of the cylinder liner 8 in this way, stress due to combustion stress, thermal expansion pressure, and the like is dispersed, difference of sectional area on the cylinder block 2 side is made generous, and sudden change of rigidity to be the reason of stress concentration is reduced.