PURPOSE: To facilitate the site welding job and make uniform the welding quality, by mounting only the torch on the traveling carriage, and welding by inclining the torch by a certain angle with respect to the welding direction.

CONSTITUTION: The shape of a torch 7 is curved, and the direction of curvature coincides with the moving direction of a traveling carriage 5, that is, the direction in which a flexible pipe 10 being let off reels around steel pipe piles 1A, 1B. The inclination α with respect to the welding direction (a) of the torch 7 is required by 70°W90° with respect to the advancing direction, and welding is performed by back- step process. The torch 7 and a welding machine 8 are connected with the flexible pipe 10 which is reeled in a manner capable of being let off onto a take-up reel 9 installed in the welding machine 8, and the flexible pipe 10 is let off or uncoiled from the take-up reel 9 as the traveling carriage 5 moves.