A lower rail of channel structure is secured a floor. The lower rail has a longitudinally extending slot in an upper portion thereof. An upper rail is slidably engaged with the lower rail and mounts thereon a seat. The upper rail includes a base portion which is slidably received in the lower rail and a supporting wall portion which extends upward from the base portion and passes through the slot of the lower rail for connection with the seat. A protecting cover covers one end portion of the lower rail and has a longitudinally extending slit into which the supporting wall portion of the upper rail is inserted when the upper rail assumes an extreme position relative to the lower rail. A concealing structure is integral with the projecting cover and constructed to conceal the slit. The concealing structure and the supporting wall portion are so constructed as to permit the insertion of the supporting wall portion into the slit irrespective of provision of the concealing structure.