A surface treatment method capable of imparting exceptional corrosion resistance and moisture resistance to an NB heat exchanger. The method includes subjecting an NB heat exchanger to a chemical conversion treatment to form a chemical conversion film on the surface thereof using a chemical conversion treatment agent that contains zirconium and/or titanium in a total amount of 5-5,000 ppm by weight, vanadium in an amount of 10-1,000 ppm by weight, and has a pH of 2-6; bringing the NB heat exchanger on whose surface the chemical conversion film is formed into contact with a hydrophilization agent containing a hydrophilic resin and a guanidine compound and/or a salt thereof; and baking the NB heat exchanger subjected to the contacting process, whereby a hydrophilic film is formed on the surface thereof.