A heat exchanger configured to exchange heat between a refrigerant flowing through a plurality of flat tubes (31, 61) and air, the heat exchanger comprising: a windward tube bank and a leeward tube bank arranged in a flow direction of the air, each of which being constituted of the plurality of flat tubes (31, 61) arranged one above the other; and fins (32, 62) jointed to the flat tubes (31, 61), wherein the windward tube bank is divided into a principal windward bank portion and an auxiliary windward bank portion, each of which being constituted of the plurality of the flat tubes arranged one above the other, the leeward tube bank is divided into a principal leeward bank portion and an auxiliary leeward bank portion, each of which being constituted of the plurality of flat tubes arranged one above the other, the heat exchanger further comprises a header collecting pipe connected to one end of each of the flat tubes constituting the principal leeward bank portion to form principal communicating spaces (75a-75f) communicating with the plurality of flat tubes, the heat exchanger being characterised by: the auxiliary windward bank portion is located below the principal windward bank portion, and is constituted of the flat tubes smaller in number than the flat tubes of the principal windward bank portion, the auxiliary leeward bank portion is located below the principal leeward bank portion, and is constituted of the flat tubes smaller in number than the flat tubes of the principal leeward bank portion, the auxiliary windward bank portion, the auxiliary leeward bank portion, the header collecting pipe, the principal leeward bank portion, and the principal windward bank portion are arranged in series in a refrigerant flow path, the refrigerant flows from the auxiliary windward bank portion to the principal windward bank portion in this order when the heat exchanger functions as an evaporator, and the refrigerant flows from the principal windward bank portion to the auxiliary windward bank portion in this order when the heat exchanger functions as a condenser. a windward tube bank and a leeward tube bank arranged in a flow direction of the air, each of which being constituted of the plurality of flat tubes (31, 61) arranged one above the other; and fins (32, 62) jointed to the flat tubes (31, 61), wherein the windward tube bank is divided into a principal windward bank portion and an auxiliary windward bank portion, each of which being constituted of the plurality of the flat tubes arranged one above the other, the leeward tube bank is divided into a principal leeward bank portion and an auxiliary leeward bank portion, each of which being constituted of the plurality of flat tubes arranged one above the other, the heat exchanger further comprises a header collecting pipe connected to one end of each of the flat tubes constituting the principal leeward bank portion to form principal communicating spaces (75a-75f) communicating with the plurality of flat tubes, the heat exchanger being characterised by: the auxiliary windward bank portion is located below the principal windward bank portion, and is constituted of the flat tubes smaller in number than the flat tubes of the principal windward bank portion, the auxiliary leeward bank portion is located below the principal leeward bank portion, and is constituted of the flat tubes smaller in number than the flat tubes of the principal leeward bank portion, the auxiliary windward bank portion, the auxiliary leeward bank portion, the header collecting pipe, the principal leeward bank portion, and the principal windward bank portion are arranged in series in a refrigerant flow path, the auxiliary windward bank portion is located below the principal windward bank portion, and is constituted of the flat tubes smaller in number than the flat tubes of the principal windward bank portion, the auxiliary leeward bank portion is located below the principal leeward bank portion, and is constituted of the flat tubes smaller in number than the flat tubes of the principal leeward bank portion, the auxiliary windward bank portion, the auxiliary leeward bank portion, the header collecting pipe, the principal leeward bank portion, and the principal windward bank portion are arranged in series in a refrigerant flow path, the refrigerant flows from the auxiliary windward bank portion to the principal windward bank portion in this order when the heat exchanger functions as an evaporator, and the refrigerant flows from the principal windward bank portion to the auxiliary windward bank portion in this order when the heat exchanger functions as a condenser.