PURPOSE: To promote work efficiency by forming circular recessed channels around bolt holes of plate-faces joined with two structural and members, fitting and projecting circular projected edges to the recessed channels to eat the projected edges into the structural in members the case bolts are fastened.

CONSTITUTION: Circular projected edges 2 and 3 are assembled to strapped plates 1. After that, circular recessed channels 2a and 3a are respectively formed in a concentric-shape together with bolt holes 4 in plate-faces on one side of the plates 1 so as to surround the peripheral surfaces of the bolt holes 4. The plate-faces 1a are turned to the insides and are brought into contact with both sides of structural members 5 and 5' butted to couple the plates 1, and when bolts 7 and 7 pass through bolt holes 4 and 4 and through-holes 6 and 6 of the structural bers 5 and 5' and fasten both strapped plates 1 and 1, the strapped plates 1 and 1 are such that the projected edges 2 and 3 are eaten into the structural members 5 and 5' and that the adherence of the plate-faces 1a and 1a to the structural members forms joining construction.