To simply, firmly, and precisely lay a rail for a sliding door in a rail groove formed in a sill and a lintel without using a pressure sensitive adhesive double coated tape and a rubber adhesive.

A rail made of rigid resin is formed such that side plates 1b and 1b are integrally arranged on the edges on both sides of a bottom plate 1a. In this case, horizontal lock protrusions 2 and 2 in the shape of saw teeth in cross section are protruded only on the outer sides of the lower ends of the two side plates 1b and 1b being the end faces on both sides of the bottom plate 1a. When the rail is fitted in a rail groove formed in a sill and lintel the horizontal lock protrusions 2 and 2 are cut through the wall surface on both sides of a groove bottom and the rail is firmly fixed. A reaction force generated through cut-through of the horizontal lock 2 is received by a bottom plate 1a to prevent the two side plates 1b and 1b from inward deformation and the opening width of the rail is kept at a constant value and simple and high-precise laying is practicable.