PURPOSE: To reduce processing time and to obtain a high quality weld zone by forming a specific angle and a truncated-conical connecting surface at one end of a pipe, making the corresponding connecting surface in a part of the thickness of the wall member, and welding from the outside of the wall so that these connecting parts coincide with each other.

CONSTITUTION: In the case of fixing a pipe 23 by welding to the thick wall member 20 having an opening 22 of a passage, one end of the pipe 23 is formed with a connecting surface 24 of a truncated-conical shape which has a cross section with a half angle of the apex angle between 30 and 60 degrees and with the size in the width direction decreasing towards the tip end of the pipe 23, and which has the axis line coincident with that of the pipe 23. A truncated conical connecting surface 25, which opens on the outer side of the wall 20 and which corresponds to the connecting surface 24 of the pipe 23 flaring externally, is formed coaxially with the opening, only in a part of the thickness of the wall member 20. Then, the connecting surface of the pipe 23 and that of the wall 20 are arranged so as to coincide with each other, and are welded from the outside of the wall along the connecting surfaces 24, 25 by means of an electron beam.