To provide a cylinder reducing reciprocating friction between a piston ring and the inner wall surface of the cylinder in a region where a piston ring slides.

In the cylinder having a piston sliding on an inner wall surface, a plurality of recess parts are formed in a stroke center region in an inner wall surface of the cylinder, which is a region from the undersurface position of a ring groove in the bottom piston ring at a top dead center to the top surface position of a ring groove in the top piston ring at a bottom dead center of the piston. When an area of the stroke center region is assumed to be 100%, the sum of areas of all recess parts falls within a range of 1-80%. At least one of the recess part is formed in each of all cross sections in a cylinder circumferential direction. No recess part is formed in the region other than the stroke center region in the inner wall surface of the cylinder.