Cutting of a corrugated fin into predetermined lengths for a radiator in a vehicle. The phase mating is, when the cutter is moved in the direction opposite to the corrugated member, effected at the correcting point where the torque in the cutter is the same as the torque at a cutting point where the corrugated member is moved synchronizingly with the cutter. Thus, automatic phase mating is effected, thereby causing the fin to be cut precisely at a trough of the fin.