To decrease damage to a concrete in the case of an earthquake in the circumference of an opening of a beam.

An opening reinforcement structure 20 for reinforcing the opening S of the beam 10 includes first and second opening reinforcing bars 21 and 22. The first opening reinforcing bar 21 includes an approximately U-shaped and hanging reinforcing section 21a and a hooking section 21b continued to a pair of upper ends of the reinforcing section 21a and hooked on a beam main bar 11 on the upper side. The second opening reinforcing bar 22 includes an approximately U-shaped and rising reinforcing section 22a and a hooking section 22b continued to a pair of lower ends of the reinforcing section 22a and hooking on a beam main bar 12 on the lower side. The opening S is surrounded by the reinforcing section 21a of the first opening reinforcing bar 21 and the reinforcing section 22a of a second opening reinforced bar 22.