To reinforce the bonding performance (rigidity and proof stress) of steel beams while being mountable to flanges with the coinciding beam depth when the span and beam height of the steel beams are different, and to safely and easily fix the steel beams at low cost with high efficiency without needing skill.

A steel column M and a frame part 6 of a column-beam joint body 5 fittingly inserted into or closely fitted to the steel column M are fixed by a one-side locking device 10. The one-side locking device with a locking means 2 insert-fitted to a tail locking bolt 1A and with a fixing nut 20 screwed, is inserted from a frame part through hole 6a toward a steel column insertion hole p. When a collar part 3b of the locking means abuts on the outside of the joint body frame part, an enlarging function part 31 of the locking means passes through the steel column insertion hole, and in this state, the fixing nut is fastened. The tail locking bolt thereby slides in a slip-out direction, and in accordance with this, the enlarging function part is compressed and enlargingly deformed to the outer periphery to form a fastening part 32 to thereby fix the steel column and the joint body frame part.