PURPOSE: To provide a high sound and vibration insulating effect and to reduce the amount of sinking of a floor.

CONSTITUTION: A dewatering-preventive gel layer 8 is provided on a building structure floor surface 2 and a gel layer 4 is provided thereon. A floor bed panel 7 is bonded to the upper surface of the gel layer 4 and a floor finishing plate 10 is installed so as to constitute a floor 12. The gel layer 4 is formed in the following way: a water-absorptive synthetic high polymer gel is dried to generate particulates of synthetic xerogel. The dewatering-preventive gel layer 8 is provided on the building structure floor surface 2 and on part of a wall surface 3 in advance. An aqueous solution having synthetic xerogel mixed therein is stirred and cast onto the dewatering-preventive gel layer 8 and then the aqueous solution of synthetic xerogel becomes an aggregate layer of water-containing particle gel. A high polymer additive for forming a network structure is sprinkled from the upper surface of the aggregate of water- containing particle gel to form a gel layer 4 having the water-containing particle gel enclosed in the network structure.