To easily and rapidly form cut-out parts, prevent the forming speed of fins from being low and maintain a good production efficiency when corru gated fins having the cut-out parts formed for reducing heat transfer are formed.

A method for forming fins comprises a corrugation working step for bending a fin material 25 in a corrugated form, a pitch adjusting step for adjusting the pitch of the corrugated fin material 25 and a cutting step for cutting the fin material 25 whose pitch is adjusted to a prescribed length. Before the corrugation working step, a cut-out part working step is provided for boring cut-out parts 24 in the fin material 25 by rotating rotors 33 ad 35 with boring mechanisms formed on their peripheries. Since the cut-out parts 24 are formed by rotating the rotors 33 and 35, the cut-out parts 24 can be bored successively at prescribed intervals without preventing the movement of the fin material 25.