PURPOSE: To prevent a subgrade from frozenly rising by spreading heat insulating ballast all over a subgrade, forming a subgrade course composed of asphalt over the aforesaid ballast, and thereby isolating low temperature over the ground.

CONSTITUTION: Heat insulating ballast 2 where synthetic resin foams 4 are hardened by hydraulic binder 5 so as to be integrated, is spread all over a subgrade 1. The insulating ballast 2 is formed by mixing the synthetic resin forms 4 in a spherical or roughly spherical shape each with the hydraulic binder 5 such as cement and the like, and concurrently by kneading fibers, sands, miscible material and the as required. Next, a subgrade course 3 composed of asphalt, concrete and the like is formed over the heat insulating ballast. In a cold district, low temperature over the ground is isolated, the subgrade is prevented from being frozen, is also prevented from swelling up, and concurrently the thermal expansion coefficient of the heat insulating ballast is made small, so that the ballast itself is thereby prevented from being damaged. By this constitution, the durability of the subgrade course can thereby be enhanced.