PURPOSE: To facilitate designing and to simplify the whole process of foundation work by providing offset grooves at a pair of shoulders of a square block made of a foamed resin material.

CONSTITUTION: The present invention is constituted of a square-shaped block body 1 which comprises a foaming resin material laid out on a ground which can be enveloped on a continuous footing of a structure provided at a specified dimensional span in a planned direction based on a design module. A gangplank to which these blocks 1 are laid out at a spacing at least on one pair of peripheral edges out of the upper and lower peripheral edges of the blocks 1 is arranged to shape up an offset-groove 3 to be installed over the blocks 1. The blocks l and the gangplank are adapted to form a virtually continuous plane in a state that the gangplank is installed to the offset grooves, which can be used as a form for a continuous footing or the like, and which can be served as a permanent form as they are. They fit well to the ground and are capable of meeting with foundations in sophisticated and various shapes as well. These blocks properly scatter floor slabs and loads working on the floor slabs, and eliminate air conditioning loss by functioning as an insulation material.