A heat exchanger (25, 25a, 25b, 25c) that is a heat exchanger in which a refrigerant that flows in from a first inlet and a second inlet exchanges heat with an air flow and flows out from an outlet, the heat exchanger comprising: an upwind heat-exchanging unit (50, 50a, 50b, 50c); a downwind heat-exchanging unit (60, 80) that, in an installed state, is disposed beside the upwind heat-exchanging unit on a downwind side of the upwind heat-exchanging unit, the downwind heat-exchanging unit having the second inlet; and a flow path formation portion (70, 75) that forms a refrigerant flow path (RP, RP2) at a location between the upwind heat-exchanging unit and the downwind heat-exchanging unit, wherein the upwind heat-exchanging unit and the downwind heat-exchanging unit each include a first header (56, 66, 86) that has a first header space (Sa1, Sb1, Sc1) formed in the first header, a second header (57, 67, 87) that has a second header space (Sa2, Sb2, Sc2) formed in the second header, and a plurality of flat tubes that are connected to the first header and the second header, that are arranged side by side in a longitudinal direction of the first header and the second header, and that allow the first header space and the second header space to communicate with each other, when the refrigerant that has flown in from the first inlet and the second inlet exchanges the heat with the air flow and flows out from the outlet as a liquid refrigerant in a subcooled state, in the upwind heat-exchanging unit, a subcooling area (SC1, SC2) that is an area in which the liquid refrigerant in the subcooled state flows is formed, and an upwind outlet-side space (A3/A6) that is the first header space or the second header space that communicates with the outlet and an upwind upstream-side space (A6/A3) that is the first header space or the second header space that is disposed on an upstream side of a flow of a refrigerant at the upwind outlet-side space are formed, and characterised by the refrigerant flow path allows a downwind downstream-side space (Sb2, Sc2) and the upwind upstream-side space to communicate with each other, the downwind downstream-side space being the second header space that is disposed on a most downstream side of a flow of a refrigerant in the downwind heat-exchanging unit. an upwind heat-exchanging unit (50, 50a, 50b, 50c); a downwind heat-exchanging unit (60, 80) that, in an installed state, is disposed beside the upwind heat-exchanging unit on a downwind side of the upwind heat-exchanging unit, the downwind heat-exchanging unit having the second inlet; and a flow path formation portion (70, 75) that forms a refrigerant flow path (RP, RP2) at a location between the upwind heat-exchanging unit and the downwind heat-exchanging unit, wherein the upwind heat-exchanging unit and the downwind heat-exchanging unit each include a first header (56, 66, 86) that has a first header space (Sa1, Sb1, Sc1) formed in the first header, a second header (57, 67, 87) that has a second header space (Sa2, Sb2, Sc2) formed in the second header, and a plurality of flat tubes that are connected to the first header and the second header, that are arranged side by side in a longitudinal direction of the first header and the second header, and that allow the first header space and the second header space to communicate with each other, when the refrigerant that has flown in from the first inlet and the second inlet exchanges the heat with the air flow and flows out from the outlet as a liquid refrigerant in a subcooled state, in the upwind heat-exchanging unit, a subcooling area (SC1, SC2) that is an area in which the liquid refrigerant in the subcooled state flows is formed, and an upwind outlet-side space (A3/A6) that is the first header space or the second header space that communicates with the outlet and an upwind upstream-side space (A6/A3) that is the first header space or the second header space that is disposed on an upstream side of a flow of a refrigerant at the upwind outlet-side space are formed, and characterised by the refrigerant flow path allows a downwind downstream-side space (Sb2, Sc2) and the upwind upstream-side space to communicate with each other, the downwind downstream-side space being the second header space that is disposed on a most downstream side of a flow of a refrigerant in the downwind heat-exchanging unit.