PURPOSE: To secure the fitting strength of a functional member and facilitate the exchanging work thereof, by providing an opening on a side wall and burying and fixing a fitting tool for the functional member at the periphery of the opening and within it.

CONSTITUTION: An opening 2 is made on a side wall in advance in a factory to arrange a fitting tool 3 for a functional member. In this case, the fitting tool 3 is buried at the periphery of the opening 2 and provided within the opening. The functional member is only fixed to the fitting tool 3 for the functional member at the construction site. And hence, the construction is simplified. The functional member is neatly contained in the opening and the finished state is well and the external appearance is ensured.