A damage-repairing method of a transition piece in an embodiment includes; an oxide removing step of removing an oxide on an outer surface 20b of an inner duct 20 of a transition piece 10 having damage; a repair material placing step of placing a brazing repair material 80 on the outer surface 20b of the inner duct 20 so as to cover the damage; and a diffusion brazing step of diffusion heat treating the inner duct 20 to repair the damage formed in the outer surface 20b. The damage-repairing method of the transition piece includes: a pressurized heat treating step of, under high pressure, heat treating the inner duct 20; a surface finishing step of scraping off the brazing repair material 80 projecting from the outer surface 20b; and a non-pressurized heat treating step of solution heat treating and aging heat treating the inner duct 20 under non-pressure.