A heat exchanger has a case housing a plurality of heat transfer tubes, a pair of header portions for water inflow and for water outflow, the pair of header portions connecting with both ends of the plurality of heat transfer tubes, and a pair of wall portions for headers respectively constituted with an auxiliary member formed separately from a side plate member of the case, the wall portions for headers constituting the pair of header portions by being assembled with the side plate member. The pair of wall portions for headers are configured to be integrally connected with each other. Thereby, production of the pair of header portions are facilitated and the production cost of the heat exchanger is reduced.