An air conditioner comprising: a refrigerant circuit that includes a heat exchanger and that is arranged to cause a refrigerant to circulate therein; and a fan that is arranged to supply an airflow to the heat exchanger, the heat exchanger comprising: a plurality of flat multi-hole tubes that are disposed in line such that a direction intersecting a direction of an airflow is a longitudinal direction of each flat multi-hole tube; a header (24, 24a) to which an end portion of each of the plurality of flat multi-hole tubes is connected; a plurality of fins that are joined to the flat multi-hole tubes, wherein a space inside the header to which the flat multi-hole tubes are connected is formed so as to cause a larger amount of a refrigerant to flow on an upstream side than on a downstream side in the direction of the airflow, wherein the space inside the header to which the flat multi-hole tubes are connected is formed so as to have a wider space that allows the refrigerant to pass therethrough on the upstream side than on the downstream side in the direction of the airflow, characterised in that a portion that faces the end portion of each of the flat multi-hole tubes on the downstream side in the direction of the airflow is provided with a concave-convex portion including a convex portion that projects toward a side of the flat multi-hole tubes and an oppositely-recessed concave portion, the portion being a part of a contour of the space inside the header to which the flat multi-hole tubes are connected. a refrigerant circuit that includes a heat exchanger and that is arranged to cause a refrigerant to circulate therein; and a fan that is arranged to supply an airflow to the heat exchanger, the heat exchanger comprising: a plurality of flat multi-hole tubes that are disposed in line such that a direction intersecting a direction of an airflow is a longitudinal direction of each flat multi-hole tube; a header (24, 24a) to which an end portion of each of the plurality of flat multi-hole tubes is connected; a plurality of fins that are joined to the flat multi-hole tubes, wherein a space inside the header to which the flat multi-hole tubes are connected is formed so as to cause a larger amount of a refrigerant to flow on an upstream side than on a downstream side in the direction of the airflow, wherein the space inside the header to which the flat multi-hole tubes are connected is formed so as to have a wider space that allows the refrigerant to pass therethrough on the upstream side than on the downstream side in the direction of the airflow, characterised in that a portion that faces the end portion of each of the flat multi-hole tubes on the downstream side in the direction of the airflow is provided with a concave-convex portion including a convex portion that projects toward a side of the flat multi-hole tubes and an oppositely-recessed concave portion, the portion being a part of a contour of the space inside the header to which the flat multi-hole tubes are connected. a plurality of flat multi-hole tubes that are disposed in line such that a direction intersecting a direction of an airflow is a longitudinal direction of each flat multi-hole tube; a header (24, 24a) to which an end portion of each of the plurality of flat multi-hole tubes is connected; a plurality of fins that are joined to the flat multi-hole tubes, wherein a space inside the header to which the flat multi-hole tubes are connected is formed so as to cause a larger amount of a refrigerant to flow on an upstream side than on a downstream side in the direction of the airflow, wherein the space inside the header to which the flat multi-hole tubes are connected is formed so as to have a wider space that allows the refrigerant to pass therethrough on the upstream side than on the downstream side in the direction of the airflow, characterised in that a portion that faces the end portion of each of the flat multi-hole tubes on the downstream side in the direction of the airflow is provided with a concave-convex portion including a convex portion that projects toward a side of the flat multi-hole tubes and an oppositely-recessed concave portion, the portion being a part of a contour of the space inside the header to which the flat multi-hole tubes are connected.