A heat exchanger including: rows of heat exchanging units that are superposed with one another in an air flow direction of the heat exchanger; and flat multi-hole tubes that extend from a first end toward a second of the heat exchanging units in a first direction in the heat exchanging units and that include gas-side flat multi-hole tubes. A refrigerant flows in the heat exchanging unit in the first direction. A number of the gas-side flat multi-hole tubes that are included in a front-most row heat exchanging unit on an airflow upstream side of the heat exchanger is less than a number of the gas-side flat multi-hole tubes included in a rear-most row heat exchanging unit on an airflow downstream side of the heat exchanger.