A rodless cylinder comprises a cylinder tube, and a slide table which is movable back and forth along the cylinder tube. A cover member is fixed to the cylinder tube in a state in which an attachment base is secured to the slide table. A dust-preventive member is integrally interposed by an inner wall of the cover member, a side wall of the cylinder tube, and a side wall of the attachment base. The dust-preventive member is provided to be capable of making elastic deformation in a direction of the arrow B perpendicular to a direction of the arrow A.