PURPOSE: To ensure the construction of a road in a weak subsoil by laminating a styrofoam body on a styrofoam plate after the styrofoam plate is laid between L type retaining walls buried in the subsoil, and placing a concrete layer thereon to form a road surface.

CONSTITUTION: A road 1 on a weak subsoil 2 is constituted of a sand layer 3, styrofoam 4, styrofoam body 14 and L type retaining walls, etc. The styrofoam plate 4 is so formed that plate pieces 5 with drain holes 4a and L-shaped plate pieces 6 are assembled in the shape of a recession. Drain ditches 4c are arranged to the bottom of the styrofoam plate 4 at a certain interval. After that, the styrofoam plate 4 is buried in the sand layer 3, the L type retaining walls 8 are arranged thereon, the styrofoam body 14 is laminated between them, a reinforced concrete layer 15 and filling 17 are piled up thereon, and it is coated with a concrete 18, etc., to form the road surface 16. According to the constitution, land subsidence of the road surface caused by the gush of underground water can be prevented.