PURPOSE: To decrease the number of parts necessary for assembling a heat exchanger and to shorten the assembling time by forming a forming plate in the state of a series of linked parallel placing development.

CONSTITUTION: The mutually adjoining plural pieces each of forming plates 11, 11 are formed in the state of a series of parallel placing developments that the mutual side edges each other is linked via one or plural pieces of slender strip like connection pieces 10. This integral forming body is manufactured once by surviving the connection piece 10 in the necessary length between the adjoining forming plates each other simultanously with the forming operation of individual forming plate 11 by pressing. In assembling, a tube element 1 part is formed by overlapping in a principal rafter state the forming plates 11, 11 each other that makes a pair each other by bending the connection piece 10 in order, and tank parts 8, 8 are overlapped each other in the state of the adjacent forming plate of the tube element 1 each other being back to back.