A plurality of angle portions 2c of the fins on an upstream side and those on a downstream side of an air flow are provided so as to be substantially symmetrical with each other. Due to this, bending forces are continuously exerted on a thin plate-like fin material in a direction where the bending deformation of the fin material is cancelled, during the fin forming process. Accordingly, when the angle portions 2c are formed it can be prevented in advance that the fin material 11 is deformed in a state where the repeated deformations of the fin material 11 are accumulated in the same direction.