PURPOSE: To provide specified loading capacity, without enlarging the whole by using a large buoyant block body having a prescribed loading capacity only for its building part at building of a building which exceeds the loading capacity of a basic float block on a basement main body.

CONSTITUTION: A basement main body 2 is formed into a flat surface rectangular shape, by sequentially arranging basic float blocks 6 lengthwise and crosswise and connecting and integrating them by a connection device 7. Thereafter, when building a building 15 which exceeds the allowable maximum loading capacity of the basic float blocks 6 at building of houses, plants or hospitals, etc., on the basement main body 2, the ones to receive live load of not less than the allowable maximum loading force out of the basic float blocks 6 are replaced with large buoyant block bodies 26 with the buoyancy of an extending part 21 to extend downward below the float blocks 6. Consequently, it is possible to secure specified loading capacity without enlarging the basement main body as a whole.