In a heat exchanger installed on an intake path or exhaust path of an internal combustion engine and configured to cool gas by transferring heat between the gas containing exhaust and refrigerant, a water-repellent portion is formed at part of a portion that contacts the gas in a gas path through which the gas flows, and a hydrophilic portion is formed at another part of the portion. The water-repellent portion is a portion provided with water-repellent treatment. The hydrophilic portion is a portion provided with hydrophilic treatment. The hydrophilic portion is formed at a part having a temperature lower than the temperature of the part at which the water-repellent portion is formed while the heat exchanger is actuated.