A header has a tank member having a transverse cross-section of a circular arc shape, and a tube attachment member having a transverse cross-section flatter than the above-mentioned circular arc shape as a whole, are opposed and joined. The tube attachment member has identical curvature portions constituted with an inner diameter R2 which is the same as the inner diameter R2 of the tank member at both sides to be joined to the tank member, and has a large curvature portion constituted with an inner diameter R1 which is larger than the inner diameter R2 of the tank member at an intermediate portion interposed between the identical curvature portions. A connecting position P between the identical curvature portion and the large curvature portion is inside from both edges of a tube.
