A fluid pressure cylinder comprising: a cylinder tube including a slide hole inside the cylinder tube ; a piston unit disposed to be reciprocable along the slide hole; and a piston rod protruding from the piston unit in an axial direction, wherein the piston unit includes: a piston body protruding radially outward from the piston rod ; a packing attached to an outer circumferential part of the piston body ; a holding member attached to the outer circumferential part of the piston body and including a magnet holding portion; and a magnet held by the magnet holding portion and disposed partially in a circumferential direction of the piston body , and wherein the magnet holding portion has a cavity (58a1) opened in an outer circumferential surface of the holding member , wherein the holding member includes a circumferential portion extending in the circumferential direction along the outer circumferential part of the piston body ; an inner circumferential surface of the circumferential portion is attached to the outer circumferential part of the piston body ; the cavity (58a1) is opened in an outer circumferential surface of the circumferential portion , characterized in that the magnet holding portion protrudes inward from the inner circumferential surface of the circumferential portion attached to the outer circumferential part of the piston body . a cylinder tube including a slide hole inside the cylinder tube ; a piston unit disposed to be reciprocable along the slide hole; and a piston rod protruding from the piston unit in an axial direction, wherein the piston unit includes: a piston body protruding radially outward from the piston rod ; a packing attached to an outer circumferential part of the piston body ; a holding member attached to the outer circumferential part of the piston body and including a magnet holding portion; and a magnet held by the magnet holding portion and disposed partially in a circumferential direction of the piston body , and wherein the magnet holding portion has a cavity (58a1) opened in an outer circumferential surface of the holding member , wherein the holding member includes a circumferential portion extending in the circumferential direction along the outer circumferential part of the piston body ; an inner circumferential surface of the circumferential portion is attached to the outer circumferential part of the piston body ; the cavity (58a1) is opened in an outer circumferential surface of the circumferential portion , characterized in that the magnet holding portion protrudes inward from the inner circumferential surface of the circumferential portion attached to the outer circumferential part of the piston body . a piston body protruding radially outward from the piston rod ; a packing attached to an outer circumferential part of the piston body ; a holding member attached to the outer circumferential part of the piston body and including a magnet holding portion; and a magnet held by the magnet holding portion and disposed partially in a circumferential direction of the piston body , and wherein the magnet holding portion has a cavity (58a1) opened in an outer circumferential surface of the holding member , wherein the holding member includes a circumferential portion extending in the circumferential direction along the outer circumferential part of the piston body ; an inner circumferential surface of the circumferential portion is attached to the outer circumferential part of the piston body ; the cavity (58a1) is opened in an outer circumferential surface of the circumferential portion , characterized in that the magnet holding portion protrudes inward from the inner circumferential surface of the circumferential portion attached to the outer circumferential part of the piston body . <EOS>