To expand an extent capable of forming a sheet metal ring by inter- plane bending.

A part corresponding to an inner peripheral side of a helical part of a band-shaped steel sheet 64 to be sent to a ring forming machine 10 to execute inter-plane bending is heated by a heater 20, after that quenching is executed through rapid cooling by a cooler 21, and thus its mechanical strength is made stronger than the strength of a part corresponding to an outer peripheral side of the helical part, and the peak value of compression stress of the inner peripheral side of the helical part by the inter-plane bending is decreased.