Heat is exchanged between exhaust gas passing through a plurality of tubes and cooling water passing through a plurality of passages defined outside of the plurality of tubes layered with each other. A heat exchanger includes a temperature decreasing portion arranged in a predetermined area on an outer surface of the tube adjacent to an inlet side of exhaust gas. The temperature decreasing portion is configured to decrease a temperature of a thermal boundary layer of the outer surface of the tube relative to cooling water by increasing a heat transmitting ratio between the outer surface of the tube and cooling water.