PURPOSE: To easily fix mutual elongated cylindrical members in a short time by welding collars having indentations and projections at their end edges to a pair of the cylindrical members and installing coupling members to the external surfaces of the two collars and holding the coupling members together against axial movement.

CONSTITUTION: A coupling member 16 for mutually coupling tubes 12 and 14 surrounds coupled collars 30, 32, and comprises two half bodies 86, 88. The half bodies 86, 88 are connected pivotally by an axial hinge 90. When the upper sections of the coupled collars 30, 32 welded at the end sections of the tubes 12 and 14 are folded so as to be surrounded and engaged, the shoulders 96, 98 of the member 16 are engaged oppositely to the shoulders 66, 68 of sleeves 30, 32, and the sleeves 30, 32 are held so as not to be separated in the axial direction from each other. The projections 60, 58, 62, 64 of the collars 30, 32 prevent the mutual rotation of the tubes 12 and 14, and integrally tune the tubes.