To safely and quickly install decorative slates on the surface of an exterior wall of a building structure to give profoundity and natural beauty thereto.

When the decorative slate consisting of approximately 12mm to 15mm in thickness, approximately 10cm to 40cm in length and approximately 30cm to 60cm in width is stuck on the surface of the exterior all of the building structure, a U-shaped groove is perforated through the inside of each center of the upper end edge and/or the lower end edge of the facing slate, the other lower end edge is bent in an L-shape, and a longer locking metal fitting forming an upward and downward or the upward projected body is made on the front end, the locking metal fitting is mounted on the surface of the exterior wall of the building structure and, at the same time, the projected body of the locking metal fitting is stuck on the surface of the exterior wall while engagedly fitting it to the inside of a U-shaped groove of the decorative slate.