A piston assembly (10A to 10F) comprising: a piston main body (13, 60, 88, 100, 120); and a piston rod joined to the piston main body (13, 60, 88, 100, 120), wherein: the piston main body (13, 60, 88, 100, 120) includes a first piston member (40, 62, 84) and a second piston member (42, 64, 86) which are constituted from plate-like members; the first piston member (40, 62, 84) and the second piston member (42, 64, 86) are joined mutually under a condition of being superimposed in an axial direction of the piston rod; a seal installation groove (17, 65, 96) that extends in a circumferential direction is formed between an outer peripheral edge of the first piston member (40, 62, 84) and an outer peripheral edge of the second piston member (42, 64, 86); wherein the second piston member (42, 64, 86) has a plate thickness over the entirety thereof, and a through hole (47, 66) is disposed in the first piston member (40, 62, 84) so as to penetrate in a plate thickness direction thereof; characterized by a fitting portion that is fitted in the through hole is formed to project on one end of the piston rod, wherein the piston rod includes a cylindrical body portion, wherein the fitting portion is formed on a proximal end of the body portion and has an outer diameter which is smaller than that of the body portion, and wherein the outer diameter thereof is the same as or slightly smaller than the inner diameter of the through hole provided on the first piston member (40, 62, 84). a piston main body (13, 60, 88, 100, 120); and a piston rod joined to the piston main body (13, 60, 88, 100, 120), wherein: the piston main body (13, 60, 88, 100, 120) includes a first piston member (40, 62, 84) and a second piston member (42, 64, 86) which are constituted from plate-like members; the first piston member (40, 62, 84) and the second piston member (42, 64, 86) are joined mutually under a condition of being superimposed in an axial direction of the piston rod; a seal installation groove (17, 65, 96) that extends in a circumferential direction is formed between an outer peripheral edge of the first piston member (40, 62, 84) and an outer peripheral edge of the second piston member (42, 64, 86); wherein the second piston member (42, 64, 86) has a plate thickness over the entirety thereof, and a through hole (47, 66) is disposed in the first piston member (40, 62, 84) so as to penetrate in a plate thickness direction thereof; characterized by a fitting portion that is fitted in the through hole is formed to project on one end of the piston rod, wherein the piston rod includes a cylindrical body portion, wherein the fitting portion is formed on a proximal end of the body portion and has an outer diameter which is smaller than that of the body portion, and wherein the outer diameter thereof is the same as or slightly smaller than the inner diameter of the through hole provided on the first piston member (40, 62, 84). the piston main body (13, 60, 88, 100, 120) includes a first piston member (40, 62, 84) and a second piston member (42, 64, 86) which are constituted from plate-like members; the first piston member (40, 62, 84) and the second piston member (42, 64, 86) are joined mutually under a condition of being superimposed in an axial direction of the piston rod; a seal installation groove (17, 65, 96) that extends in a circumferential direction is formed between an outer peripheral edge of the first piston member (40, 62, 84) and an outer peripheral edge of the second piston member (42, 64, 86); wherein the second piston member (42, 64, 86) has a plate thickness over the entirety thereof, and a through hole (47, 66) is disposed in the first piston member (40, 62, 84) so as to penetrate in a plate thickness direction thereof; characterized by a fitting portion that is fitted in the through hole is formed to project on one end of the piston rod, wherein the piston rod includes a cylindrical body portion, wherein the fitting portion is formed on a proximal end of the body portion and has an outer diameter which is smaller than that of the body portion, and wherein the outer diameter thereof is the same as or slightly smaller than the inner diameter of the through hole provided on the first piston member (40, 62, 84).