Around a damaged area of a jacket of a flexible tube, a top coat layer whose adhesion to a polymer layer is weak is peeled off with an adhesive tape. The damaged area and its immediate surrounding area are sanded, and then wiped with ethanol for cleaning. A main agent and a hardening agent are heated and defoamed while being mixed to prepare a repair material. The repair material is applied to the damaged area and the immediate surrounding area. A heat shrink tube is opened along slits in an end portion and covers the flexible tube. The heat shrink tube is shrunk with hot air, pressing the repair material and making it flush with a surface of the jacket. The flexible tube is heated with an electric hot plate to harden the repair material. Thereafter, the heat shrink tube is peeled off from the flexible tube.