An air conditioner comprising: a heat exchanger including a plurality of heat transfer units arranged in a third direction that is different from both of a first direction and a second direction, the second direction intersecting with or being perpendicular to the first direction; and a fan configured to generate airflow flowing in the second direction and passing through spaces between the heat transfer units, wherein the heat transfer units are each divided into an airflow-upstream region and an airflow-downstream region in the second direction, characterized in that: in each of the heat transfer units, a plurality of heat transfer channel portions and a plurality of auxiliary heat transfer portions are formed, each of the heat transfer channel portions extending in the first direction, each of the auxiliary heat transfer portions extending in the first direction, the heat transfer channel portions being arranged in the second direction, the auxiliary heat transfer portions being arranged in the second direction; when used as an evaporator, the heat exchanger is configured to cause a refrigerant to flow into the heat transfer channel portions disposed in the airflow-upstream region, and then cause the refrigerant to flow out to the heat transfer channel portions disposed in the airflow-downstream region; and the number of the heat transfer channel portions disposed in the airflow-downstream region is larger than the number of the heat transfer channel portions disposed in the airflow-upstream region. a heat exchanger including a plurality of heat transfer units arranged in a third direction that is different from both of a first direction and a second direction, the second direction intersecting with or being perpendicular to the first direction; and a fan configured to generate airflow flowing in the second direction and passing through spaces between the heat transfer units, wherein the heat transfer units are each divided into an airflow-upstream region and an airflow-downstream region in the second direction, characterized in that: in each of the heat transfer units, a plurality of heat transfer channel portions and a plurality of auxiliary heat transfer portions are formed, each of the heat transfer channel portions extending in the first direction, each of the auxiliary heat transfer portions extending in the first direction, the heat transfer channel portions being arranged in the second direction, the auxiliary heat transfer portions being arranged in the second direction; when used as an evaporator, the heat exchanger is configured to cause a refrigerant to flow into the heat transfer channel portions disposed in the airflow-upstream region, and then cause the refrigerant to flow out to the heat transfer channel portions disposed in the airflow-downstream region; and the number of the heat transfer channel portions disposed in the airflow-downstream region is larger than the number of the heat transfer channel portions disposed in the airflow-upstream region. in each of the heat transfer units, a plurality of heat transfer channel portions and a plurality of auxiliary heat transfer portions are formed, each of the heat transfer channel portions extending in the first direction, each of the auxiliary heat transfer portions extending in the first direction, the heat transfer channel portions being arranged in the second direction, the auxiliary heat transfer portions being arranged in the second direction; when used as an evaporator, the heat exchanger is configured to cause a refrigerant to flow into the heat transfer channel portions disposed in the airflow-upstream region, and then cause the refrigerant to flow out to the heat transfer channel portions disposed in the airflow-downstream region; and the number of the heat transfer channel portions disposed in the airflow-downstream region is larger than the number of the heat transfer channel portions disposed in the airflow-upstream region.