Abstract: When forming a circular tube-shaped header pipe by rolling a plate material, surfaces extending at a right angle to the internal and external surfaces of the plate material are formed at the two side edges extending in the lengthwise direction, and the plate material is rolled in such a manner that the two side edges face opposite each other while setting the two side edges in a non-contact state. In this process, the gap between the two side edges is set at or less than 0.3 mm. In order to manufacture this header pipe, a plate material that has been cut to a specific length is trimmed in the direction of its short side to achieve a specific dimension and also at a right angle to the front and rear surfaces of the plate material. After a piping insertion hole is formed, the entirety of the plate material is rolled so that its cross section achieves a U-shape.