PURPOSE: To provide a manufacture of a heat exchanger which produces an effect of corrosion prevention, of course, and also eliminates a phenomenon of blue water, pitting corrosion of I and II types or erosion and others.

CONSTITUTION: This manufacture of a heat exchanger comprises an instantaneous gas water heater. In the heat exchanger wherein copper tubes are provided in a casing and a large number of heat exchange fins are juxtaposed at the parts of the copper tubes positioned inside the casing, a plating process of forming a tin plating layer on the inner peripheral surface of the copper tube by circulating an electroless tin plating solution through the copper tube is conducted (step 14) after an assembling process (steps 11, 12).