Abstract: Lock plate portions are formed in a front end portion of a flat plate portion of a first rail. A stopper member is mounted on a flat plate portion of a second rail. A lock hole and elastic strips are provided on a sloping plate portion of the stopper member. The lock hole is disposed such that it faces the lock plate portions when the second rail moves in the forward direction to a predetermined position. When the lock plate portion faces the lock hole, the elastic strip displaces a front end portion of the sloping plate portion to the flat plate portion side using elastic force of the elastic strip itself and elastic force of the sloping plate portion. Accordingly, the lock plate portions are relatively received in the lock hole.