To improve the heat transfer performance of a heat exchanger and achieve an air conditioner having high energy efficiency. The heat exchanger 15 includes a plurality of fins 1 that is arranged in parallel with each other with a predetermined spacing along the rotational axis direction of a blower 5; heat exchanger tubes 2 that are substantially perpendicularly inserted into the fins 1 so as to form a plurality of rows along the longitudinal direction of the fins 1 connected to each other along the airflow direction, to thereby form refrigerant channels; and a branch portion that is provided to connection portions of the heat exchanger tubes 2, and that partially increases or decrease the number of paths in the refrigerant channels. Herein, the refrigerant flowing through each of a plurality of the refrigerant channels passing through paths mutually different at least at one portion between the refrigerant inlet and the refrigerant outlets, flows along one direction from the windward-side row to the leeward-side row, or from the leeward-side row to the windward-side row in the airflow direction, in sequence between rows. Here, one-path portion is provided in the most windward-side row heat exchanger tubes. Furthermore, the fins 1 in close contact with a refrigerant outlet 18 in the case when the heat exchanger 15 is operated as a condenser, and a connection piping 16c are thermally separated by separation means 21.