PURPOSE: To fold up a decorative plate without generating traces of a fold or a crease on a decorative sheet.

CONSTITUTION: A shallow groove 4 is provided on the surface side of a base plate 2 so as to be in parallel with opposed both sides, a notched line 5 is provided on the back side of the base plate 2 on the same position as the groove 4, and the base plate 2 is folded at the notched line so that a decorative sheet 3 is inside the folding direction. The folded part is relieved into a space formed with the groove 4, so as not to generate a fold or a crease on the decorative sheet 3. Hereby, the width is shortened so as to be convenient for custody and transport, efficiency of stretchedly mounting operation is improved, and the decorative sheet 3 can be stretchedly mounted on a beautiful finish without generating traces of a fold or a crease on the decorative sheet 3.