A heat exchanger , comprising: a front-row heat exchange element and a rear-row heat exchange element ; the front-row heat exchange element includes a plurality of stacked flat tubes ; fins provided on the flat tubes ; and front row headers erected in a stacking direction in which the flat tubes are stacked, and connected to the flat tubes , wherein the front-row heat exchange element is configured to function as an evaporator that causes heat exchange between air and a refrigerant flowing into the flat tubes through the headers , to evaporate the refrigerant, the rear-row heat exchange element includes a plurality of stacked flat tubes ; fins provided on the flat tubes ; and rear row headers erected in a stacking direction in which the flat tubes are stacked, and connected to the flat tubes , the rear-row heat exchange element is configured to function as an evaporator that causes heat exchange between air and a refrigerant flowing into the flat tubes through the headers , to evaporate the refrigerant, the front-row heat exchange element and the rear-row heat exchange element are in use arranged in the air flowing direction with the front-row heat exchange element arranged in a front row (F) located on an upstream side of the flow of the air and the rear-row heat exchange element arranged in a rear row (R) located on a downstream side of the flow of the air, characterised in that a flow path cross-sectional area (A f ) of each of front-row headers is smaller than a flow path cross-sectional area (A r ) of each of rear-row headers , the heat exchanger being configured such that a flow velocity of the refrigerant flowing through each of the front-row headers is higher than a flow velocity of the refrigerant flowing through each of the rear-row headers . a front-row heat exchange element and a rear-row heat exchange element ; the front-row heat exchange element includes a plurality of stacked flat tubes ; fins provided on the flat tubes ; and front row headers erected in a stacking direction in which the flat tubes are stacked, and connected to the flat tubes , wherein the front-row heat exchange element is configured to function as an evaporator that causes heat exchange between air and a refrigerant flowing into the flat tubes through the headers , to evaporate the refrigerant, the rear-row heat exchange element includes a plurality of stacked flat tubes ; fins provided on the flat tubes ; and rear row headers erected in a stacking direction in which the flat tubes are stacked, and connected to the flat tubes , the rear-row heat exchange element is configured to function as an evaporator that causes heat exchange between air and a refrigerant flowing into the flat tubes through the headers , to evaporate the refrigerant, the front-row heat exchange element and the rear-row heat exchange element are in use arranged in the air flowing direction with the front-row heat exchange element arranged in a front row (F) located on an upstream side of the flow of the air and the rear-row heat exchange element arranged in a rear row (R) located on a downstream side of the flow of the air, characterised in that a flow path cross-sectional area (A f ) of each of front-row headers is smaller than a flow path cross-sectional area (A r ) of each of rear-row headers , the heat exchanger being configured such that a flow velocity of the refrigerant flowing through each of the front-row headers is higher than a flow velocity of the refrigerant flowing through each of the rear-row headers . <EOS>