Abstract: On a roadbed layer (4) is formed a cushion layer (5) of 50 mm to 200 mm in thickness which is formed by using elastic chips alone or in mixture with sand, each elastic chip being formed of elastic material and having a size of 7 mm to 0.05 mm, preferably 2 mm to 0.5 mm and a specific gravity of 1.8 to 3.5. The cushion layer (5) contains elastic chips of 3 wt % or more. The elastic chips are formed by mixing a material selected from the group consisting of synthetic rubber, natural rubber and synthetic resin with a specific gravity adjusting material of inorganic powder having a specific gravity of 3 or more.