A heat exchanger for use as an evaporator includes a heat exchange core having a plurality of heat exchange tubes arranged in a left-right direction at a spacing, and a refrigerant turn tank as a lower tank disposed toward a lower end of the heat exchange core. The heat exchange tubes are inserted through respective tube insertion holes formed in the turn tank and joined to the tank. The turn tank has drain grooves each extending from a forwardly or rearwardly outer end of each of the tube insertion holes for discharging condensation water to below the turn tank therethrough. Each of the drain grooves has a bottom extending gradually downward as the bottom extends away from the tube insertion hole. the exchanger is used as the evaporator, the top surface of the turn tank can be drained of water with an improved efficiency.