A heat exchanger used as an evaporator includes a refrigerant inlet header section having a refrigerant inlet at a right end portion, and a plurality of heat exchange tubes connected to the refrigerant inlet header section. The interior of the refrigerant inlet header section is divided by a partition plate into an upper space into which refrigerant flows via the refrigerant inlet and a lower space into which the heat exchange tubes project. A communication opening is formed in the partition plate on the left side of a heat exchange tube at the left end so as to establish communication between the upper and lower spaces. A guide portion is formed at a left edge portion of the communication opening such that the guide portion projects from a lower surface of the partition member into the lower space so as to guide the refrigerant toward the right end side.