PURPOSE: To make welding between steel tube easy with providing a desired thermal expansion margin, further to be suitable for successively welding a large number of tubes, also to make welding possible with surely securing the desired thermal expansion margin.

CONSTITUTION: A combustible spacer made 4 of cardboard, etc., is placed on the flat face 12 in the radial direction of a tube 1 of the stepped part 11 formed on the inner peripheral face of the groove part of the tube 1. A thickness of the spacer 4 is the desired thermal expansion margin (a). Successively, the groove part 20 of the steel tube 1 is inserted in the stepped part 11, the spacer 4 is sandwiched between the groove end 21 of a steel tube 2 and flat face 12. Subsequently, a steel tube 2 is lightly pressed on the steel tube 1 side, as this sandwiched condition is maintained, the end part 13 of groove of the steel tube 1 and the outer peripheral face of the steel tube 2 are welded.