A heat exchanger comprising: at least one heat exchanger unit including a plurality of vertically arranged flat tubes, fins joined to the flat tubes, a first header collection pipe (340, 370) connected to one of ends of each of the flat tubes, and a second header collection pipe (345, 380) connected to the other end of each of the flat tubes, the heat exchanger unit being divided into a plurality of vertically arranged heat exchange regions (35, 37) (37, 135, 235) (335, 337, 365, 367), and the plurality of heat exchange regions (35, 37) (37, 135, 235) (335, 337, 365, 367) being connected in series when the heat exchanger functions as an evaporator, the heat exchanger allowing a refrigerant flowing through the flat tubes and air to exchange heat, wherein each of the heat exchange regions (35, 37) (37, 135, 235) (335, 337, 365, 367) is further divided into a plurality of vertically arranged heat exchange sections, communicating spaces each communicating with the plurality of flat tubes are formed in each of the first and second header collection pipes (40, 70) (340, 345, 370, 380) in a one-to-one relationship with the heat exchange sections, and in two of the heat exchange regions connected to each other when the heat exchanger functions as an evaporator, a downstream one (135, 235) (335, 365, 367) of the heat exchange regions has not less heat exchange sections than the heat exchange sections of an upstream one (37, 235) (337, 365, 367) of the heat exchange regions, and characterised in that a most downstream one of the heat exchange regions has more heat exchange sections than a most upstream one of the heat exchange regions. at least one heat exchanger unit including a plurality of vertically arranged flat tubes, fins joined to the flat tubes, a first header collection pipe (340, 370) connected to one of ends of each of the flat tubes, and a second header collection pipe (345, 380) connected to the other end of each of the flat tubes, the heat exchanger unit being divided into a plurality of vertically arranged heat exchange regions (35, 37) (37, 135, 235) (335, 337, 365, 367), and the plurality of heat exchange regions (35, 37) (37, 135, 235) (335, 337, 365, 367) being connected in series when the heat exchanger functions as an evaporator, the heat exchanger allowing a refrigerant flowing through the flat tubes and air to exchange heat, wherein each of the heat exchange regions (35, 37) (37, 135, 235) (335, 337, 365, 367) is further divided into a plurality of vertically arranged heat exchange sections, communicating spaces each communicating with the plurality of flat tubes are formed in each of the first and second header collection pipes (40, 70) (340, 345, 370, 380) in a one-to-one relationship with the heat exchange sections, and in two of the heat exchange regions connected to each other when the heat exchanger functions as an evaporator, a downstream one (135, 235) (335, 365, 367) of the heat exchange regions has not less heat exchange sections than the heat exchange sections of an upstream one (37, 235) (337, 365, 367) of the heat exchange regions, and characterised in that a most downstream one of the heat exchange regions has more heat exchange sections than a most upstream one of the heat exchange regions.