PURPOSE: To facilitate work and to make sure of fitting by providing interlocking recesses having slopes to the inside on one side of an insertion joint section of cylindrical members, providing hooking steps to the outsides on the outer side, and jointing both recesses and steps with one touch.

CONSTITUTION: When a cover 2 is mounted to cylindrical gateposts 1 or when cylindrical column materials or beam materials are inserted into corner joint to joint, interlocking projections (interlocking recesses) 6 consisting of slopes 6a and hooking surfaces 6b are formed to the insides of a female side member 1 by plastic deformation. Hooking steps 7 for interlocking with the interlocking projections (recesses) 6 are provided to a male side member 2. When the cover 2 is mounted to the gateposts 1, the hooking steps 7 of the cover 2 are pushed in by sliding the slopes 6a of the interlocking projections, the hooking steps 7 are interlocked with the hooking surfaces 6b, they are slightly elastically compressed, and they are fixed with one touch operation. The interlocking projection 6 is so formed that a tool having an edge section similar to the projection is inserted from the end and is pushed with oil pressure. By the constitution, a fitting section without slackness or slippage can be easily obtained.