A hydraulic fluid cylinder is equipped with a cylinder tube, a piston unit, and a piston rod. The piston unit has a gasket and a piston body comprising a plurality of members and provided with a gasket-mounting groove. The piston body has a first piston member which projects to the outside from the piston rod, and also has a second piston member positioned so as to be adjacent to the first piston member. The gasket-mounting groove is formed from a combination of at least two members.