PURPOSE: To form a spacious basement by arranging a plurality of building units at spaces, and connecting both the building units through a perpendicular panel.

CONSTITUTION: Two basement building units 30, 30 are opposite in each long side and put side by side at a space and connected through a perpendicular panel 10 for forming a basement 3. In each of building units 30, 30, a plurality of fixtures 12 is provided on the columns 31 of mutually opposite side faces in their vertical directions, and in the panel 10, a nut is previously buriedly provided on one side face on an indoor side, and in a job site, a fixing bolt is inserted from the side of the fixture 12 and threadedly engaged with the nut for attaching the panel 10 to the column 31. An inner wall 35 is provided on the the inside of the panel 10, and spaces between the two sheets of panels 10 and the building unit 30 are also used as an underground space. Thus a spacious basement can be formed of a few basement building units.