PURPOSE: To enable to enhance heat-exchanging efficiency, by treating the heat- transmitting surfaces of a heat exchanger to enhance the emissivity thereof.

CONSTITUTION: An inner pipe 6 made of iron and having a thickness of 0.5mm is subjected to Unichrome treatment of inner and outer surfaces thereof, and the surfaces are coated with an acrylic matting black film 9. As a result, the average surface emissivity in a temperature range of 0W100°C becomes about 0.95. Accordingly, a heat-exchanging efficiency which can not be attained by a conventional heat exchanger can be achieved by a simple treatment.