To provide stronger attachment by fixing together a ceiling panel to a coupling member with fixing screws, cramping the upper portions of wall panels with a holding piece and a fixing piece of the coupling members and by fixing the wall panel and coupling member with fixing screws.

A side end surface of a ceiling panel 2 is stuck and contacted to a positioning piece 4d of a coupling member 4, and the ceiling panel 2 and the coupling member 4 are fixed with fixing screws 6 through insertion hole 7 of a horizontal base piece 4a of the coupling member 4. Next, the ceiling panel 2 is placed on the top end of the wall panel 1, a holding piece 4b and a fixing piece 4c of each coupling member 4 are fitted to the upper end portion of the wall panel 1, a fixing screw 6 is screwed from the inner side of a bathroom through an insertion hole 7, and a coupling member 4 is fixed to the wall panel 1. In addition, a linking projection 9 of a waterproof cover 5 is linked to the linking projected stripe 8 of a fixing piece 4c, and upper and lower elastic tongue pieces 10 are contacted under pressure elastically to the ceiling panel 2 and the wall panel 1. Thus, sufficient waterproof treatment can be performed for the jointing portion.