To provide a rodless cylinder preventing an outer peripheral surface of a cylinder tube from causing partial abrasion in the peripheral direction, even when a clearance is arranged between an insertion hole of a slider and an outer peripheral surface of an outside magnet.

The clearance is arranged between the insertion hole 4a of the slider 4 and the outer peripheral surface of the outside magnet 5, and the outside magnet 5 is movably arranged in the radial direction to the insertion hole 4a. Since the outside magnet 5 can be held on the central side of the insertion hole 4a by an elastic material 6 by holding the outside magnet 5 in the insertion hole 4a via the elastic material 6 arranged between the insertion hole 4a and the outside magnet 5, an inner peripheral surface of the outside magnet 5 does not partially deviatedly contact in the peripheral direction with the outer peripheral surface of the cylinder tube 1, and the abrasion slanted in the peripheral direction of the cylinder tube 1 can be reduced.