PURPOSE: To set directly and with high precision a squareness or a working margin between a tube axis and one end face of a flange, as a preliminary stage for joining the flange to the tube end by welding, and to enable the positioning of both.

CONSTITUTION: After a spacer 7 is fitted in the chuck pawl 5 of a chuck 4, a tube end 10 is held by the chuck pawl 5. At this time, the tube end 10 is pressed against the side of a flange working face 55 of the chuck 4 through the spacer 7. Since the axial line common to the three chuck pawls 5 and the flange working face 55 form a right angle, a tube axis L-L and one end face of the flange form a right angle. The trace of removal of the spacer 7 becomes a working margin for storing a welding thickness.