A cylinder includes a curved cylinder body, a first support member, and a piston assembly. The curved cylinder body defines a gas chamber passing through opposite ends. The first support member includes a support housing slidably mounted on the outer circumferential surface of the cylinder body and a first permanent magnet securely mounted in the support housing. The piston assembly received in the gas chamber includes a first piston member and a second piston member rotatably coupled to the first piston member. The first and second piston members respectively have a second permanent magnet which is arranged to be attracted to the first permanent magnet. When the piston assembly moves along the curved cylinder body from the first end to the second end in the gas chamber, the first support member moves with the piston assembly on the outer circumferential surface of the curved cylinder body.

A magnet-type rodless cylinder (1) is provided with a guide mechanism having an outer rolling groove (26), an inner rolling groove (29), a cylinder body (32), a pair of connection paths (34) for connecting an area between the outer rolling groove and the inner rolling groove with the cylinder body, and a plurality of steel balls (36) that can roll within an endless circuit (35) formed of the cylinder body, the connection paths and the area between the outer rolling groove and the inner rolling groove, wherein the connection paths (34) are formed at a pair of inner members, i.e. return caps (33), disposed on the inner sides of metal end plates (12) that are attached to the front and rear ends in the sliding direction of a sliding body (4).