A heat exchanger (10) includes a first flat pipe (1) having a plurality of through-holes through which a high-temperature fluid flows, a second flat pipe (2) having a plurality of through-holes (21) through which a low-temperature fluid flows, a first inlet header (3) having a tubular shape and connected to one end of the first flat pipe (1), a first outlet header (4) having a tubular shape and connected to other end of the first flat pipe (1), a second inlet header (5) having a tubular shape and connected to one end of the second flat pipe (2), and a second outlet header (6) having a tubular shape and connected to other end of the second flat pipe (2). The first flat pipe (1) and the second flat pipe (2) are stacked such that flat surfaces thereof are in contact with each other. The low-temperature fluid flowing into the through-holes (21) in the second flat pipe (2) from the second inlet header (5) is a fluid in a two-phase gas-liquid state. The low-temperature fluid flows into the through-holes (21) in the second flat pipe (2) from the second inlet header (5) in a substantially horizontal direction or in an upward direction relative to the substantially horizontal direction.