In the present invention, a method for manufacturing a heat exchanger includes an assembly step and a film-forming step. In the assembly step, multiple components (21, 22) constituting a heat exchanger are assembled. The film-forming step is carried out after the assembly step, and films (30, 31, 32, 33, 34, 35) are formed by CVD on the surfaces of the multiple components (21, 22) constituting the heat exchanger. The films are capable of suppressing the formation of through-holes arising from corrosion. Since the film-forming step is carried out after the assembly step, it is possible to prevent the films from being scratched during delivery or assembly. Since the films are formed through CVD, it is possible to prevent the occurrence of clogging in an intricate section in the heat exchanger during the film-forming step.