To provide a floor structure of superior workability and high load- resistance.

This building substrate 1 used for a floor substrate part or a wall substrate part is constituted of a steel plate 2 formed into a bend cross sectional shape, fixing members 3 which are attached along the both side edges 2a of the steel plate 2 and fixed to a floor framing or a framework side of the building, and a heat insulating part 4 which is formed by filling urethane foam material into a part surrounded by the steel plate 2 and the fixing member 3. The substrate 1 is directly attached to the floor framing or the framework of the building such as a sleeper with the steel plate 2 facing the inside of the building. This constitution can dispense with a floor joist in a floor substrate part so as to improve workability and shorten the construction term.