To provide a heat exchanger having high cooling efficiency with a small pressure loss and capable of discharging moisture from an inside tube, and to provide a method for manufacturing the heat exchanger.

An EGR cooler 1 serving as a heat exchanger cools exhaust gas G flowing through an inside tube 2, with a cooling fluid C flowing through an outside tube 3. The center axes of small diameter parts 12a, 13d and a large diameter part 13a in the inside tube 2 are decentered so that the required top parts of the small diameter parts and the required top part of the large diameter part are aligned in an axial direction and that the required top part of a plate part 21 abuts on the required top part of the large diameter part. The plate part and a connection part are formed in substantially conical shape in which the penetrating positions of the center axes of the small diameter parts are projected in the same direction, and the conical top part in the plate part and a portion, abutting on the large diameter part, of the plate part are provided with drip holes h1, h2, respectively. The heat exchanger high in cooling efficiency with a small pressure loss and preventing corrosion caused by moisture can thereby be provided.