A heat exchanger (1) has a tank (10) for accommodating a heat medium therein and a tube (3) connected to the tank (10) through a core plate (4). Specifically, an end portion of the tube (3) is inserted into a connection hole (40) of the core plate (4) and is mechanically joined thereto. A joined portion of the tube (3) and the core plate (4) is covered with adhesive (5) through a wettability improving film (6) for improving wettability of the adhesive (5) to the joined portion. As a result, a sealing property between the tube (3) and the core plate (4) is easily and stably improved by the adhesive (5).