A heat exchanger is provided to allow a partition to be firmly bond to a tank by inserting an upper end of the partition into a partition insertion groove formed in the tank. A heat exchanger comprises a plurality of tubes(30) forming a heat exchange medium communication passage. A plurality of heat radiation fins(40) are interposed between the tubes. A header(20) has a plurality of tube insertion holes into which both ends of the tube are inserted. A tank(10) is coupled with an upper portion of the header to form a heat exchange medium passage. At least one partition(22) is integrally formed with the header. The tank is formed with a partition insertion groove(11) into which the upper end of the partition is inserted. A leakage check hole is formed in the partition insertion groove.
