To provide a reinforcement structure by which an opening of a reinforced concrete beam can be reinforced without working on complicated bar arrangement or spending much time and labor.

The reinforcement structure 10 of the opening 12 of the reinforced concrete beam 20 comprises an opening reinforcing member 11 which is cylindrical and fittingly inserted into the opening 12, and shear reinforcements 15 including first reinforcing bars 13 which are provided with bent parts, respectively, bending around beam main reinforcements 16 including at least the beam main reinforcements 16 provided on both lateral ends of the reinforced concrete beam 20, and are joined to an outer peripheral surface of the opening part reinforcing member 11, and second reinforcing bars 14 which are partly overlapped with a plurality of the first reinforcing bars 16 and are arranged astride a plurality of beam main bars 16.