PURPOSE: To elevate the reliability and welding efficiency of a joining part by welding without performing the weld by upward posture for the circumference welding joining each other a large-sized thick steel pipe in a fixed condition.

CONSTITUTION: An I-shaped groove part 7 is formed with opposing the pipe ends 1a, 1b of steel pipes 1A, 1B. A bead 11 is then supperposed with repeated movement horizontally in the arrow mark direction of a welding torch 3 from the six o'clock position VI of the lowest part with the downward posture from the inner face side of the steel pipe by an arc weld of the circular arc part A at the lower side which is between four o'clock position IV and eight o'clock position VIII in a circumference direction. The part B of the space between two o'clock position II and ten o'clock position X is then welded with repeated movement in a horizontal direction of the torch 3 respectively from the position X and position II with doward posture from the outer face side of the steel pipe. Then, the part C of between the four o'clock position IVand the two o'clock position II and the part D of between the ten o'clock position Xand the eight o'clock position VIII are subjected to a vertical welding of upward advance or downward advance with the torch 3.