a preparation step for preparing a cylindrical member having male threaded portions formed at the ends, the cylindrical member being imparted with a cylindrical form by bringing opposing end parts of each of a first half tube and a second half tube to face each other;

a screwing step in which linking members, which have female threaded portions to be screwed onto the male threaded portions, are screwed onto the male threaded portions; and

a diffusion-joining step in which during the tightening produced by the screwed-on linking members, heating is performed in a state where the opposing end surfaces of the linking members and the cylindrical member are pressed against each other and in a state where the opposing end parts of the first half tube and the second half tube are pressed against each other, the opposing end surfaces of the linking members and the cylindrical member being diffusion-joined and the opposing end parts of the first half tube and the second half tube being diffusion-joined.