On the surface of a heat exchanging portion in a heat exchanger, a film made of a polyaniline is formed. When the heat exchanger operates, condensed water is adhered on the surface of the heat exchanger, and contacts the film made of the polyaniline. Accordingly, oxygen dissolved in the condensed water becomes active oxygen by the polyaniline, and organic substances contained in the condensed water is decomposed. Thus, an amount of organic substances attached to a surface of the heat exchanger can be reduced in low cost without equipment extension.