The purpose of the present invention is to provide a heat exchanger capable of suppressing boiling and lime precipitation due to local overheating of a heating medium and at the same time improving heat transmittance efficiency. The heat exchanger of the present invention for achieving said purpose comprises: heating medium flow paths, through which a heating medium flows, inside a plurality of laminated plates; and a heat exchange unit in which combustion gas flow paths, through which a combustion gas flows, are alternately formed to be adjacent to each other, wherein the heat exchange units have a heating medium differential distribution unit which enables the flow rate of the heating medium to be differentially distributed to parallel heating medium flow paths of the heat exchange unit such that the flow rate of the heating medium passing through the parallel heating medium flow paths is balanced with the quantity of absorbed heat transmitted to the plates.