One cylindrical pipe is inserted into the hollow cylindrical portions of first and second holders made of hard plastic, and the other cylindrical pipe is inserted into the hollow semi-cylindrical portions. The first and second pipe holders are butted together at butt surfaces, respectively. The cylindrical pipe inserted into the butted hollow cylindrical portions is held in the hollow cylindrical portions using a clamping tool, and the cylindrical pipe inserted into a cylindrical portion that becomes hollow cylindrical by butting is held in the hollow semi-cylindrical portions using the clamping tool.