In a rodless cylinder, a moving member which is moved along a cylinder body is reduced in height for the sake of compactness and stability. A stopper is mounted on a stopper support base, which is also reduced in height to preclude the possibilities of obstructing movement of the moving member, and includes a stopper lock mechanism with an adjusting screw suitable for use with the stopper support base of low height. More specifically, in a rodless cylinder having a stopper support base mounted on top of a cylinder body, the stopper support base is provided with inclined surfaces at the opposite ends thereof symmetrically in face to face relation with downwardly-facing inclined surfaces formed on opposite lateral side portions of the cylinder body, and firmly fastened to the cylinder body by setting bolts and clamp members having inclined surfaces face to face with the inclined surfaces. The stopper is threaded into the stopper support base on the cylinder body and is locked in position by threading the stopper into a lock plate and fastening the lock plate to the stopper support base by bolts.