To straighten the curl of a metallic thin strip formed for the time in custody and to restrain the development of twist, warp, waving, etc., caused by strain, by arranging a brake device and a forming roll device, in order.

The metallic thin strip 11 is firstly passed through the brake device 13. In the brake device 13, one pair of brake rolls attaching a mechanism freely settable to pushing pressure of the rolls and a brake mechanism developing a constantly controlled brake force by connecting the brake rolls are provided, and the metallic thin strip 11 passed through between the brake rolls, receives the braking force. Successively, the metallic thin strip 11 passed through the brake device 13, is supplied to the forming roll group 14. The forming roll group 14 is formed into a prescribed shape to the metallic thin strip 11 while applying the feeding force. As a result, in the metallic thin strip 11 pased through the brake device 13 and the forming roll group 14, the tension is developed from the difference between the braking force and the feeding force.