A sternum reinforcing device to be used after a sternotomy or a sternal fracture, made of a biocompatible material, comprises an elongated modular member (10) with a pair of legs (20, 30), that are joined each other by a body portion (4), and a pair of arms (6, 7). The legs (20, 30) can be fitted in an intercostal space of the thorax of a patient, laterally to the sternum, and can be bent in a mutually opposite direction, on the internal side of the thorax, after being fitted therein. The arms (6, 7), extending from one side to the other of the body portion (4) and orthogonally to the pair of legs (20, 30), constitute a male part and a female part, respectively, of a prismatic type coupling for consecutive modular members.