A heat exchanger 1 includes at least a plurality of tubes 2 and an upper tank 4 communicating with an upper end portion of a tube group formed by the tubes 2 so as to distribute coolant along the up/down direction. The coolant flows in through an inflow port 9 located at the upper tank 4 in the heat exchanger 1. An open-top coolant intake guide passage 25 is disposed at the inflow port 9 and the coolant intake guide passage 25 is thus inserted at the upper tank 4. With this configuration, coolant flowing at a very low flow rate has improved distribution, and uniformity in the output air temperature is achieved, while ensuring that the structure does not create excessive resistance against the coolant flow at a high flow rate.