PURPOSE: To secure the flatness of the surface for welding a waterproof sheet of a disk installed to the concrete coating surface of a tunnel through a nailing and the like, installed by overcoming a bad condition to be an irregular surface of the concrete coating surface.

CONSTITUTION: To the first part 11 forming the central disk fixed to the concrete coating surface 2 of a tunnel, the second part 12 forming the peripheral disk whose necessary side surface is placed to the surface for welding a waterproof sheet 4 is positioned at the inner side of a tunnel pit inner than the first part 11. And they are connected by a flexible connector 13 which obtains the flexibility by its thin wall or flexible property.