PURPOSE: To aim at preventing a relative displacement between modules and breakage thereof during earth quake by arranging a plurality of support columns between a building having an underwater movable structure and a ballast tank, and by arranging an elevating device which can be extended and retracted in accordance with concavities and convexities in the water bottom.

CONSTITUTION: Concrete paths 1 are laid on bridge piers 4 and beams 5 arranged on a base 3 on the sea bottom 2 so that the paths 1 run in every direction, and a base 9 incorporating a ballast tank is laid below an upper building 7 through the intermediary of a plurality of support columns 8. Further, jacks 10 are disposed between the building 7 and the support columns 8, an underwater hallway 60 which is moored between movable structures 50 by means of anchors 11 and lanyards 12 is suspended. Further, houses and hallways composed of the underwater structures 50 as modules which are combined together arbitrarily at the forward ends of paths extending from the land are assembled so as to form one structure. With this arrangement, a water-surface building can be set horizontally irrespective of concavities and convexities in the water bottom.