A heat exchanger having a plurality of heat transfer tubes for heat exchange between refrigerant flowing inside the plurality of heat transfer tubes and air flowing outside the plurality of heat transfer tubes, the heat exchanger comprising: an auxiliary heat exchange unit having a first auxiliary heat exchange region, and a second auxiliary heat exchange region facing the first auxiliary heat exchange region in a flow direction in which the air flows; and a main heat exchange unit having a first main heat exchange region, a second main heat exchange region facing the first main heat exchange region in the flow direction, a third main heat exchange region disposed opposite to the first auxiliary heat exchange region across the first main heat exchange region, and a fourth main heat exchange region facing the third main heat exchange region in the flow direction and disposed opposite to the second auxiliary heat exchange region across the second main heat exchange region, wherein the plurality of heat transfer tubes of each of the first auxiliary heat exchange region and the second auxiliary heat exchange region are fewer than the plurality of heat transfer tubes of each of the first main heat exchange region, the second main heat exchange region, the third main heat exchange region, and the fourth main heat exchange region, the first auxiliary heat exchange region, the first main heat exchange region, and the third main heat exchange region are disposed windward of the second auxiliary heat exchange region, the second main heat exchange region, and the fourth main heat exchange region, respectively, in the flow direction, the auxiliary heat exchange unit and the main heat exchange unit are configured to cause the refrigerant to flow successively through the first auxiliary heat exchange region, the second auxiliary heat exchange region, the first main heat exchange region, the second main heat exchange region, the fourth main heat exchange region, and the third main heat exchange region when the heat exchanger functions as an evaporator, the main heat exchange unit has a fifth main heat exchange region disposed between the first main heat exchange region and the third main heat exchange region, and a sixth main heat exchange region disposed between the second main heat exchange region and the fourth main heat exchange region, and the main heat exchange unit is configured to cause the refrigerant to flow successively through the first main heat exchange region, the second main heat exchange region, the fifth main heat exchange region, the sixth main heat exchange region, the fourth main heat exchange region, and the third main heat exchange region when the heat exchanger functions as the evaporator. an auxiliary heat exchange unit having a first auxiliary heat exchange region, and a second auxiliary heat exchange region facing the first auxiliary heat exchange region in a flow direction in which the air flows; and a main heat exchange unit having a first main heat exchange region, a second main heat exchange region facing the first main heat exchange region in the flow direction, a third main heat exchange region disposed opposite to the first auxiliary heat exchange region across the first main heat exchange region, and a fourth main heat exchange region facing the third main heat exchange region in the flow direction and disposed opposite to the second auxiliary heat exchange region across the second main heat exchange region, wherein the plurality of heat transfer tubes of each of the first auxiliary heat exchange region and the second auxiliary heat exchange region are fewer than the plurality of heat transfer tubes of each of the first main heat exchange region, the second main heat exchange region, the third main heat exchange region, and the fourth main heat exchange region, the first auxiliary heat exchange region, the first main heat exchange region, and the third main heat exchange region are disposed windward of the second auxiliary heat exchange region, the second main heat exchange region, and the fourth main heat exchange region, respectively, in the flow direction, the auxiliary heat exchange unit and the main heat exchange unit are configured to cause the refrigerant to flow successively through the first auxiliary heat exchange region, the second auxiliary heat exchange region, the first main heat exchange region, the second main heat exchange region, the fourth main heat exchange region, and the third main heat exchange region when the heat exchanger functions as an evaporator, the main heat exchange unit has a fifth main heat exchange region disposed between the first main heat exchange region and the third main heat exchange region, and a sixth main heat exchange region disposed between the second main heat exchange region and the fourth main heat exchange region, and the main heat exchange unit is configured to cause the refrigerant to flow successively through the first main heat exchange region, the second main heat exchange region, the fifth main heat exchange region, the sixth main heat exchange region, the fourth main heat exchange region, and the third main heat exchange region when the heat exchanger functions as the evaporator.