PISTON

PROBLEM TO BE SOLVED: To further reduce a frictional force of a piston in a cylinder piston mechanism. SOLUTION: A skirt portion 18 of a piston 14 has a cross-sectional shape profile within a plane perpendicular to an axial direction being substantially the same along the axial direction. The cross-sectional shape profile includes: a central region having a predetermined central ellipticity defined beforehand, in a range of a predefined central region length B in a portion in a thrust direction and anti-thrust direction relative to a cylinder bore 12; both side regions having a both sides portion ellipticity larger than central ellipticity, in ranges of both sides of the central regions; and a connection region in which the central region and the both side regions are connected, a clearance with an inner diameter of a cylinder bore 12 has a central connection portion clearance defined beforehand in a connection point with the central region, a clearance from the inner diameter of the cylinder bore 12 has a both sides connection portion clearance C1 defined beforehand in a connection point with the both side regions. COPYRIGHT: (C)2011,JPO&INPIT