A method for manufacturing a flat pipe 1 with concave-convex shapes on inner and outer surfaces includes making incisions with an expanded shape of the flat pipe 1 in a metal thin plate 2 such that intermediate portions 4 (middle portions) in the expanded shape of the flat pipe 1 widthwise of the expanded shape are left uncut from the thin plate 2, machining the concave-convex shapes in a range of the expanded shape of the flat pipe 1 and raising the expanded shape of the flat pipe 1 from the thin plate 2 as cut-and-raised pieces 8 with the uncut intermediate portions 4 in between, closing tips of the cut-and-raised pieces 8 together to form an overall shape of the flat pipe 2, and separating the uncut intermediate portions 4 from the thin plate 2 to obtain the flat pipe 1.