A heat exchanger used as an evaporator is configured such that two heat exchange tube groups, each composed of a plurality of heat exchange tubes, are provided between a pair of header tanks, while being separated from each other in a front-rear direction. Each of the header tanks includes two header sections. Each header tank includes a first member to which the heat exchange tubes are connected, a second member which is joined to the first member and covers the side of the first member opposite the heat exchange tubes, and a partition plate disposed between the first and second members and having partition portions which divide the interiors of the two header sections into respective upper and lower spaces. Through holes are formed in the partition portions so as to establish communication between the upper and lower spaces of the header sections.