PURPOSE: To simplify execution by fixing rail-like members having projected threads up and down to a skeleton and, at the same time, loading inorganic surface plates having recessed grooves on up and down edges to fill joints with calkings.

CONSTITUTION: Rail-like members 4 having an upper projected thread 5 and a lower projected thread 6 are fixed to a skeleton at a specific interval. While, the surface of a kiln system inorganic material requires mirror plane plane treatment to provide recessed grooves 2 and 3 on up and down edges of the surface plates 1 applying a weatherproof coating material on them. After that, the surface plates 1 are loaded to the rail-like members 4, and joints are filled with calking materials to complete an external wall. According to the constitution, wallboards having designing efficiency can be easily provided.