To improve the durability of a fluid cylinder by expanding a slide member in the direction of an inner circumferential surface of a cylinder tube.

A first slide member 5 and a second slide member 6 are urged in the expanding direction by the elasticity of a cushion ring 10 as an elastic member. Even when the outer circumferential surfaces of the first slide member 5 and the second slide member 6 in contact with the inner circumferential surface of the cylinder tube 1 is worn, half rings 5a, 5b to constitute the first slide member 5 are moved in the direction of the inner circumferential surface of the cylinder tube 1 along an inclined surface 3c of a first guide member 3. Half rings 6a, 6b to constitute the second slide member 6 are moved in the direction of the inner circumferential surface of the cylinder tube 1 along an inclined surface 4c of a second guide member 4, and need not be exchanged frequently like a piston seal.