PURPOSE: To install a branching tube without interfering a flow of fluid by pulling out a steel ball to outside of a mold from a through-hole, warping a part of the metallic tube in the periphery of the hole with the steel ball arranging the shape of the warping port of metallic tube as the mold, and forming a collar around the hole.

CONSTITUTION: A form 4 is installed on the periphery of a hole of a metallic tube A. The form 4 is provided with a through-hole 41 for a steel ball 2 to pass through. A pull 5 of a pulling device above the form is engaged with a catch 21 and pulled upward. As a result, the steel ball 2 is raised, pushing up the periphery of the hole 1 of the tube A, pressing it tightly against the through-hole 41 of the form 4. The steel ball 2 is further raised and pulled out above the form 4. After that, a transporting tool 3 and the form 4 are removed, so that the tube A with the collar is obtained. The collar 6 thus formed is used for installing a branching tube. When a branching tube is connected with the collar 6 inserted in it, the tip end of the branching tube is prevented from projecting inside the metallic tube A, making a flow of fluid smooth without a narrowed part.