To manufacture a high quality heat exchanger by preventing a heat deformation arising with a welding heat at the time of heat welding in manufacturing a heat exchanger wherein the other heat exchanger constructing members such as a connecting pipe and the like are heat welded to the heat exchanger body.

In this method, a cooling jig 10 being composed of a cooling medium accommodation body 11 having a cooling medium inflow opening 12 and a discharge opening 13, and at least one wall face of which being a cooling face is used. When the heat welding is conducted, the cooling face of the cooling jig 10 is brought in to contact with a nearby part of a welding part B under a state that the cooling medium is flowing to the discharge opening 13 from the inflow opening 12 so as to carry out the welding while cooling the nearby part of the welding part.