PURPOSE: To surely prevent each footing of floor posts sufficiently from being moistened as well as to simplify its construction by placing age hardening material over the underfloor ground surface and in the recessed section of each footing of floor posts, and thereby integrally constructing earth floor over each footing of floor posts and its periphery.

CONSTITUTION: Powdery material 7 which will be the main component of water glass, is scattered over the underfloor ground surface 3 and in the recessed section 5 of each footing of floor posts in such a way as to be formed into a layer constant in thickness, and water is splayed 8 over the powdery material 7 thereafter so as to be left as it is until it is hardened so that a moisture prevention layer 9 is thereby formed in advance over the underfloor ground surface 3 and in the recessed section 5 of each footing of floor posts. Age hardening material 11 composed of mortar concrete and the like is placed over the moisture prevention layer 9, and earth floors 13, what they call, are integrally constructed with age hardening material 11 together with each footing of floor posts 12 and its periphery.