PURPOSE: To improve the weldability of a spiral fin and the productivity for coiling the spiral fin.

CONSTITUTION: A fin slitting device 4 is composed of a pair of cutter wheels 6 for fin slitting, a fin tension adjusting cylinder 7, a guide roller 8 and a guide roller slide rail 9. The guide roller 8 is arranged before the fin welding point, and this guide roller 8 is made to a structure slidable in the fin feeding direction. The guide roller 8 is supported by the air cylinder 7 and the fin is always imparted with certain strength of tension. An inclining cam 18 is attached to the bracket of the guide roller 8, and the rotary driving of the hydraulic motor 19 of the fin slitting cutter wheel 6 is instantaneously synchronized with the speed of fin coiling by a plunger system flow rate adjusting valve 17 in contact with the inclining cam 18.