A heat exchanger for use as an evaporator 1 comprises a heat exchange core having a plurality of heat exchange tubes 12 arranged in a left-right direction at a spacing, and a refrigerant turn tank 3 as a lower tank disposed toward a lower end of the heat exchange core. The heat exchange tubes 12 are inserted through respective tube insertion holes 53 formed in the turn tank 3 and joined to the tank 3. The turn tank 3 has drain grooves 56 each extending from a forwardly or rearwardly outer end of each of the tube insertion holes 53 for discharging condensation water to below the turn tank 3 therethrough. Each of the drain grooves 56 has a bottom extending gradually downward as the bottom extends away from the tube insertion hole 53. When the exchanger is used as the evaporator 1, the top surface of the turn tank 3 can be drained of water with an improved efficiency.