PURPOSE: To aim at preventing seizure and damage between both circumferential surfaces from occurring, by forming a curvature center of a spherical surface of a piston hear part so as to be situated in a position inside a cylinder all the time at the time of its reciprocation, while constituting an outer circumferential surface of the piston so as to press an inner circumferential surface of the cylinder with uniformly distributed force.

CONSTITUTION: In a spherical 23 forming a spherical head part of each piston 8, its curvature center O is formed so as to be situated in the more rearward then a front end part 24 of a cylinder block 6 even in a state that the piston 8 is mostly projected from the cylinder block 6. With this positional constitution, force F acts on the piston 8 from a swash plate 11 in a direction of the curvature center O of the spherical surface 23 whereby if lateral force f is produced there, this lateral force f occurs inside a cylinder 7 so that such moment as rotating the piston 8 as in the past is in no case produced, and it acts on as uniformly distributed force for pressing an outer circumferential surface of the piston 8 to an inner circumferential surface of the cylinder 7. Therefore, seizure and damage between both circumferential surfaces aforesaid are kept back, and the leak and pulsation of a hydraulic fluid are preventable from occurring, thus durability in a machine is well improvable.