To reduce pressure loss on a heat-exchanger fluid while downsizing a heat exchange and reducing the production cost of the heat exchanger without impairment of the heat transfer performance of the heat exchanger by forming a fluid channel in surfaces of thin metal plates such as stainless steel plates through the use of an etching technique or the like and by improving the shape of the fluid channel. In a heat exchanger in which a plurality of heat exchanger fins are provided in thin metal plates by using an etching technique or the like and a fluid channel for a heat-exchanger fluid is formed between the two opposed thin metal plates by alternately stacking the thin metal plates, the area of the fluid channel, through which the fluid flows between the heat exchanger fins, is made substantially uniform by forming the heat exchanger fins so as to have a curved cross-sectional shape from the front end thereof to the rear end.