An outer peripheral sealing surface of an inner surface of a core plate of a header tank is configured into a loop and extends along an outer peripheral edge portion of the core plate and clamps a packing in cooperation with an outer peripheral end portion of a tank main body of the header tank. A transition section of the outer peripheral sealing surface connects between a primary section and a secondary section, which are located in two different planes, respectively, and the plane of the secondary section is the same as a plane of a boundary portion sealing surface held between two tube connecting surfaces in the core plate.