PURPOSE: To reduce frictional resistance by providing rollers in both sides of a piston along the surface of sway movement of a connecting rod and providing rolling contact between the piston and a cylinder.

CONSTITUTION: Rollers 3 are pivotably disposed in both sides of a piston 2 in a plane of sway movement of a connecting rod 4. The respective rollers 3 are supported by a pin 7 fixed to the piston body to slightly protrude from the side of the piston 2 so that rolling contact between the piston 2 and the inner surface of a cylinder 1 can be provided by the rollers 3 to reduce frictional resistance.