To prevent corrosion or damage caused by termite, resulting from use for a long time and increase the durability of the foundation structure of a building, by using a base material made of a foaming synthetic resin instead of wooden materials as a sill.

A base material 1 is put on a continuous footing 2 as a sill and fixed thereto with anchor bolts 3 to constitute a foundation structure of a building. The base material 1 is made of a foaming synthetic resin, in which 1.5-3.0 pts.wt. of a foaming agent, 20-30 pts.wt. of wooden powder with 60-100 mesh particle size, 15-30 pts.wt. of a filler such as calcium carbonate, stone powder, or the like, and other materials such as plasticizer, stabilizer, foaming assistant, ultraviolet ray absorber, pigment, etc., are mixed in particular in 100 pts.wt. of high density polyethylene when necessary and it is molded by a molding machine. Reinforcing materials such as glass fiber or whisker may be integrally buried. In this way, the foundation structure has an extremely small water-absorption ratio in comparison with wooden materials and prevents corrosion caused by mold, bacteria, or the like or damage caused by termite or other harmful insects and increases the durability.