Residual stress generated in strip metal W is removed by annealing the strip metal W which has gone through a pressing process, and a compressing process is implemented thereafter whereby, as portions hardened due to work-hardening (in particular, folded portions 121) can be softened, the strip metal W which has gone through the pressing process can be easily compressed so that the radius of curvature of the folded portions 121 is reduced to a predetermined dimension, thereby making it possible to improve the dimensional accuracy of the finished fin 120 even if the fin 120 is made of a metal having a relatively high hardness (stainless steel).