To bring about shortening of a construction period and a reduction in construction cost by enabling an underground habitable room to be easily and rapidly constructed even within a restricted small garden, and enable execution of work for the underground habitable room even in land wherein water gushes out.

The underground habitable room is provided with a mat foundation 3, which is carried into an excavated hole 1 formed in ground so as to be fixed horizontally, and an underground habitable room structure 4 placed for fixation on the mat foundation 3. The structure 4 has a staircase 25 for ascent to/descent from an above-ground building and a cover body 26 for an emergency escape, which can be opened/closed from inside the structure 4. In addition, the structure 4 is coupled for fixation to the mat foundation 3 by means of a coupling means 41.