PURPOSE: To eliminate the weld of the branch part and the connecting part and to miniaturize the space of the connecting part by working one part of the plane sheet into plural tubes like shape in projecting and forming to the cylindrical shape with the connecting part formed on the outer side.

CONSTITUTION: The projecting part 3a to be worked by deep drawing is worked on the central of the width direction of the plane sheet 1h and in the longitudinal direction, and the connecting part 3 is formed by cutting its top end part. The plane sheet 1h is worked to the cylindrical shape with the connecting part 3 formed on the outer side, its connecting surfaces are welded and the branch part 2 is formed. Therefore, there is no part to be welded on the coolant branch, without the influence on other brazing part by the heat transfer, the space for the connection part is almost needless, so the miniaturizing of the heat exchanger can be realized.