A heat exchanger has partitioning means for dividing a header tank such that a first space and a second space of a tank main body are arranged in a longitudinal direction of the header tank. An annular outer peripheral seal surface is provided around a tube bonding surface of a core plate of the header tank over an entire perimeter thereof and is provided with a gasket. A partitioning seal surface is provided to the tube bonding surface at a position corresponding to the partitioning means, and is provided with the gasket. The gasket seals between the core plate and the partitioning means. The partitioning seal surface is positioned on a plane identical with a plane of the outer peripheral seal surface. A part of the gasket, which is held by the core plate and the tank main body therebetween, has a uniform thickness.