To reduce a load on a piston while suppressing generation of turbulent flow and vortex by arranging a flow port on a piston rod, and forming it in a taper shape so as to increase its diameter toward a radial direction outside.

A flow port of a taper shape, namely, a countersink 14 is formed on a piston rod 12. The countersink 14 is formed at a taper angle of 20° to 80° with respect to a surface perpendicular to the rod 12. The angle is an angle capable of forming an optimal flow condition, and generation of vortex and stop of flow are effectively suppressed. The countersink 14 is functioned as a peephole S, and friction and flow disturbance are not also reduced, but an abrasion condition of a ball 18 is observed by means of visual check without eliminating a piston head. Inclining flow of a radial direction is generated by the countersink 14, and a side surface load is suppressed in a minimum limit by the inclining flow on lowering of a piston.