Abstract: A slide rail used in a seat, which is of such type having an upper rail of generally inverted U-shaped section and a lower rail fitted in the upper rail slidably. In the opened area of such upper rail, there is provided a reinforcing member, in a manner extending transversely therein. In both lower rail and reinforcing member, plural lock holes are formed, and through all lock holes is engaged or disengaged a lock pawl associated with locking mechanism, for locking or unlocking the two rails, which provides a positive locking effect and robust structure.