A heat exchanger includes a first heat exchanging portion including first and second flat tubes stacked in parallel with each other and spaced from each other to allow fluid to pass between the first and second flat tubes, and a second heat exchanging portion including third and fourth flat tubes stacked in parallel with each other, spaced from each other to allow fluid to pass between the third and fourth flat tubes, and oriented crosswise to a direction in which the first and second flat tubes are oriented. The second heat exchanging portion is arranged downstream of the first heat exchanging portion with respect to flow of the fluid.