To rapidly insert an outer cylinder of a tube group into a body and center it, and to reduce number of coupling fittings by half by providing guide rails on an inner periphery of the body and a guide fixture for guiding along the rail on an outer periphery of the cylinder of the group.

Channel type guide rails 10 are mounted at a predetermined interval in parallel with an axis on an inner periphery of a body l. Square plate type guide fitments 9 are mounted at a predetermined axial interval and a circumferential interval opposed to the rails 10 on an outer periphery of an outer cylinder 2 of a tube group. When the cylinder 2 is hoisted in the body 1 at the time of assembling, the fitments 9 of a lower end are engaged with the grooves of the rails 10, and dropped to a predetermined position along the grooves. If there is a gap between the hole and the fitment 9 to be fluctuated, a wedge 11 is hit fixedly as needed. The position of the fitment is previously decided so naturally center the cylinder 2 with a tube plate tube hole.