PURPOSE: To erase a bending at both ends of neighboring waterproof sheets generated in the welding time, by installing a band form plate to a disk in order to heat weld on the inner side surface of a tunnel pit, penetrating the disk vertically in the diameter direction, and movable in its longitudinal direction.

CONSTITUTION: A band form plate 2 is provided penetrated vertically to a disk 1 in the diameter direction, and movable in the longitudinal direction of the plate 2. Then, the disk 1 is installed to a concrete coating surface D through an adequate means such as a nailing using nails 3 at the ends of the band form plate 2, and a waterproof sheet B is heat welded to the inner surface of the tunnel pit. The opposite ends in the tunnel longitudinal direction of neighboring waterproof sheets B heat welded to the disk 1, and extended on the coating surface D are welded by a runnable welding machine. After the side ends C and C are welded, the necessary side of the waterproof sheets B and B is pulled to the necessary side, so as to move the disk 1 to the same side on the plate 2, and a bending generated to the waterproof sheets B in the welding time by the welding machine is to be erased.