PURPOSE: To simply operate to fit or detach a ceiling panel from the indoor side so as to assemble it in a short time and surely fix it, and enable a seal work between a side wall panel and a ceiling panel to be omitted.

CONSTITUTION: A fixing part 13 fitted and fixed to the peripheral edge part of a ceiling panel 2, and a fixture 4 made of synthetic resin forming an engagingly fitting part 15 fitted to a catching part 6 continuously provided from the upper end part of side wall panel 5 into an arc integratedly from a fixing part 3 are fitted and fixed to the peripheral edge part of the ceiling panel 2. The arc-like engagingly fitting part 15 is hooked to the catching part 6 so as to fix the ceiling panel 2 to the upper end part of the side wall panel 5. The part of the fixture 4 made of synthetic resin contacting with the upper end part of the side wall panel 5 is formed out of a soft member so as to form a seal part 14 between the ceiling panel 2 and the side wall panel 5.