PURPOSE: To improve the adhesion without lowering an impact absorbing property by placing a primer layer on one side of a foamed thermoplastic resin sheet and laminating an asphalt sheet, heated at a specified temperature, on the primer layer of the foamed thermoplastic resin sheet.

CONSTITUTION: A primer layer 13 is placed on one side of a foamed thermoplastic resin sheet 1, and an asphalt sheet 3, which is heated at 20-200°C, is laminated on the primer layer of the foamed thermoplastic resin sheet 1. For an underground multipurpose duct 2, for example, an outer surface is covered with an asphalt-sand sheet 3, and its surface is heated by a gas burner 4. The primer layer 13 of the foamed resin sheet 1 is laminated on the surface of the asphalt sand sheet 3, and they are bonded by using a roller 5. The pit around the duct 2 is then filled with earth and sand.