To provide a structure for reinforcing a unit building, which can also be easily applied during remodeling.

In a structure for reinforcing a unit building U, a plurality of building units 1a, 1b, 1c and 1d are assembled. A ceiling beam 2 and a floor beam 3, which configure the building units 1a, 1b, 1c and 1d, are made of channel steel having recesses directed into the building units 1a, 1b, 1c and 1d. A reinforcement member 9 is arranged along the longitudinal direction of the ceiling beam 2 on the opening side of the recess of the ceiling beam 2. A web part 2a of the ceiling beam 2 and the reinforcement member 9 are fixed by means of drill screws 7... as fastening members via a connection member 6.