the present invention relates to a heat exchanger in which a plate - shaped cooling medium flow portion provides an internal cooling medium flow path inside by laminating two flat plates subjected to drawing and a cooling fin are alternately laminated , a cooling medium inlet for allowing a cooling medium to flow into the cooling medium flow path and a cooling medium outlet for allowing the cooling medium passing through the cooling medium flow path to flow out are formed in said two flat plates , and the cooling medium flowing from the cooling medium inlet to the cooling medium flow path is passed through said cooling medium flow path and is then allowed to flow out of the cooling medium outlet . according to the present invention , a bulged portion protruding on the cooling medium flow path side is formed in the cooling medium flow portion by denting at least any one of these two flat plates from the outside , and a plurality of elliptical or oval cylindrical portions whose major diameter is oriented in the flow direction of the cooling medium are provided between these two flat plates by butting the top portion of this bulged portion to the opposite flat plate . additionally , the number of the cylindrical portions is gradually decreased as the cooling medium flows downstream in the flow direction of the cooling medium .