A heat exchanger shunt comprising: a plurality of flat pipes having a plurality of refrigerant flow paths; and a pair of header pipes (3a, 3b) each of which connects both ends of the flat pipes, wherein the header pipes each include a partition plate (9a, 9b, 9c) which divides the plurality of flat pipes into a plurality of heat exchange sections (8a, 8b, 8c, 8d), a first refrigerant piping is provided to an upper portion of one header pipe of the header pipes, while a second refrigerant piping is provided to a lower portion of the one header pipe so that refrigerant outflows from the first refrigerant piping into the second refrigerant piping when the heat exchanger functions as an evaporator, the other header pipe of the header pipes includes a partition wall which divides a connection-side space of the flat pipes and a non-connection-side space of the flat pipes in a refrigerant outflow section from which the refrigerant outflows to the plurality of flat pipes, the partition wall includes a plurality of communication holes (16a, 16b) arranged in a vertical direction, and one communication hole of the communication holes has a smaller opening area than an opening area of another communication hole of the communication holes, which is in use immediately below the one communication hole characterized in that the other header pipe is provided with a damming plate, which has an updraft hole and which is provided between the plurality of communication holes, so that the heat exchange section in the other header pipe is divided into a plurality of heat exchange sections and the updraft hole is located at the non-connection-side space. a plurality of flat pipes having a plurality of refrigerant flow paths; and a pair of header pipes (3a, 3b) each of which connects both ends of the flat pipes, wherein the header pipes each include a partition plate (9a, 9b, 9c) which divides the plurality of flat pipes into a plurality of heat exchange sections (8a, 8b, 8c, 8d), a first refrigerant piping is provided to an upper portion of one header pipe of the header pipes, while a second refrigerant piping is provided to a lower portion of the one header pipe so that refrigerant outflows from the first refrigerant piping into the second refrigerant piping when the heat exchanger functions as an evaporator, the other header pipe of the header pipes includes a partition wall which divides a connection-side space of the flat pipes and a non-connection-side space of the flat pipes in a refrigerant outflow section from which the refrigerant outflows to the plurality of flat pipes, the partition wall includes a plurality of communication holes (16a, 16b) arranged in a vertical direction, and one communication hole of the communication holes has a smaller opening area than an opening area of another communication hole of the communication holes, which is in use immediately below the one communication hole characterized in that the other header pipe is provided with a damming plate, which has an updraft hole and which is provided between the plurality of communication holes, so that the heat exchange section in the other header pipe is divided into a plurality of heat exchange sections and the updraft hole is located at the non-connection-side space.