A heat exchanger includes a plurality of flat heat transfer tubes and a header, wherein, in an interior portion of each of the plurality of flat heat transfer tubes, a plurality of upwind side channels and a plurality of downwind side channels are formed, the header includes a main body portion in which an interior portion space connected to the plurality of upwind side channels and the plurality of downwind side channels is formed, a partition member that divides the interior portion space into an upwind side space, and a flow inlet/outlet port that supplies a refrigerant to a lower part of the upwind side space, and, at a lower part of the partition member, a lower communication path that communicates the downwind side space and the upwind side space is formed.