To increase strength by forming a pedestal ring by laminating an upper ring consisting of a flexible elastic material and a lower ring composed of a material having large rigidity.

The pedestal ring 3 is constituted by integrally laminating an upper ring 31 made up of a polyolefin resin such as a high-density polyethylene resin and a lower ring 32 consisting of resin concrete. A flange section 22 is fitted to the fitting section 311 of the upper ring 31 in the protective hat 2, the cover 13 of a drainage basin 1 is covered with the protective hat 2 and the drainage basin 1 is buried into a ground. Unequal load by impact load or repeated load working to the hat 2 and ground subsidence, etc., is relaxed mainly by the upper ring 31 and level led and burdened to the lower ring 32. Accordingly, the deformation and breaking of the pedestal ring 3 can be prevented while the pedestal ring 3 can resist large load, and durability can be improved.