A reinforcement for reinforcing a joint at which two lengths of an optical-fiber cable consisting of at least one optical fiber are spliced by fusion welding, such that a spliced stripped part of each optical fiber and adjacent coated parts of the cable are secured to the reinforcement by an adhesive material. The reinforcement is formed of a transparent crystallized glass which has a major crystalline component consisting of Li.sub.2 O, Al.sub.2 O.sub.3 and SiO.sub.2 and which has a low coefficient of thermal expansion. The reinforcement has a contact surface which is roughened by machining and at which the reinforcement contacts the optical-fiber cable. A strengthened layer is formed by ion exchange, on an entire surface of the reinforcement, and an adhesive layer is formed on the rough contact surface, to secure the optical-fiber cable to the contact surface, upon heating of the adhesive layer, such that the adhesive material serves to smooth the rough contact surface, and thereby renders the rough contact surface transparent.