To make welding practicable, and at the same time to prevent leakage and shortcircuit accidents by cutting off a ground water to jet during the welding of a casing.

An air seal material 5 is composed of an inside packing member 6 and an outside packing member 6. A ring-shaped holder 8 to keep a space between the packing member 6, 7 is inserted between the upper and lower edges of the packing materials 6, 7, and the holder 8 and the upper and lower ends of the packing members 6, 7 are inserted in the groove part 16 of a fitting seat 10 and fixed. The fitting seat 10 is provided with an inserting through hole 11 whose diameter is slightly larger than a boring rod 3 and an air supply port 21. The boring rod 3 is passed through the inserting through hole 11, and the air seal material 5 is arranged in a gap between the boring rod 3 and a casing 1. An compressed air is then supplied from the air supply port 21 into the inside of the air seal material 5 to make the air seal member 5 expand for tightly sticking the packing material 6, 7 to the boring rod 3 and the casing 1, and the ground water to jet from a gap between the boring rod 3 and the casing 1 can thereby be stopped.