To provide a heat exchanger which has large heat capacity and is compact by laminating plates for inner fins to high density, and to prevent respective plate parts from being clogged with solder owing to soldering.

Fin plates 5 made of thin and long metal plates are laminated up in a thickness direction, and a height direction is matched with an internal height of a flat heat exchanger body 4. Then only both height-directional end faces of each fin plates 5 and the internal surface of a body plate 1 constituting the heat exchanger body 4 are soldered and fixed, and the fin plates 5 are not soldered to each other.