According to the present invention, a panel-shaped first metal body (2) comprises a space-forming part (2a) and an overlap part (2b) that is continuous with the space-forming part (2a). A panel-shaped second metal body (4) comprises a space-forming part (4a) and an overlap part (4b) that is continuous with space-forming part (4a). When the first metal body (2) and the second metal body (4) have been assembled, a housing space layer (S1) that is for housing a sheet-shaped glass wool material 3 is formed between the space-forming parts (2a, 4a), and the overlap parts (2b, 4b) overlap. A solid joining part (W1) that connects the overlap parts (2b, 4b) is formed between the overlap parts (2b, 4b).