To provide a building foundation structure capable of reducing the settlement or the inclination of a building lower than the conventional method and to provide the building foundation structure capable of reducing a construction cost or a construction period of time lower than the conventional method.

The construction method of the building foundation structure is so constituted that a civil engineering sheet 2 is constructed on a ground leveling surface 1, gravel 3 is spread all over the civil engineering sheet 2, a dry mortar 4 is spread all over the gravel 3, and then, foaming resin blocks 5, 6 and 6' are constructed thereon to constitute a lower foundation structure and that an upper foundation structure 7 is formed on the lower foundation structure.