PURPOSE: To provide a heat insulating effect and to improve rigidity, by a method wherein, by the use of a sandwitch panel in which surface panels are laminated to the two surfaces of a light core, such as honeycomb, foamable resin, an indoor inner surface, such as floor, wall, ceiling, is formed.

CONSTITUTION: A body part 2 of an underground housing 1 is formed by reinforced concrete and is buried in an underground. An indoor inner surface is formed with a floor part material 3, a wall part material 4, and a ceiling part material 5, and all connections are connected so as to hold air-tightness by a proper means. Each member is plural unit members 3a, 4a, and 5a, made of a sandwitch panel, which are formed by adhering surface plates to both surfaces of a light core, such as honeycomb core, foamable resin, serving as a core. This prevents the occurrence of dew condensation to a wall surface and the like, enables increase of humidity, and permits sharp improvement of living comfort.