Abstract: A rail unit comprises a fixed rail, a movable rail slidably engaged with the fixed rail and a locking mechanism for locking the movable rail relative to the fixed rail. In order to increase a mechanical strength of a given part of lower flanges of the movable rail where locking openings for the locking mechanism are formed, the given part is integrally formed at an upper end thereof with a raised portion, or the lower flanges of the movable rail are formed at mutually mating portions with depressed portions. When the movable rail and the fixed rail are subjected to a mutually pulled apart movement upon a vehicle collision of the like, the raised portion or the depression portions are brought into abutment with an inner surface of the fixed rail to suppress or at least minimize deformation of the rail unit.