To easily bring about a dirt-floor finished surface and the mounting of a steel-frame column by using die steel as a footing beam in a building, in terms of a dirt-floor foundation.

A ditch section 2 is formed in leveled and rolled ground by excavating foundation sections on a front side F of the building and a backside B thereof; a footing beam support 5 is erected in the ditch section 2; an integrated frame structure 10, which is composed of a footing-beam-cum-form 9, is constituted through not only the front side F and the backside B but also side surface sides S and S of the building; and concrete 22 is placed up to a height of an upper edge of the frame structure 10.