PURPOSE: To massproduce a product having excellent quality without causing buckling and other troubles, by adopting an up-loop system as a loose coiling.

CONSTITUTION: A coil 2, the surface of which is previously treated in a surface treating tank, is horizontally delivered from horizontal pinch rolls 18, and is introduced to a coiling device 19 after being twisted by approximately one revolution. In this case of twisting, an uncoiled strip is posed against an imaginary cylinder, slanted from its lower end placed at the side of horizontal pinch rolls 18 toward its higher other end placed at the side of the coiling device 19, to spirally surround the cylinder from the surroundings of its lower end to that of its upper end, and the strip is continuously coiled by twisting the horizontal strip-surface around the vertical axis. In this case, the tight coil 2 is uncoiled from the lower surroundings to pose the inner peripheral surface of the coil 2 to be the upper surface side 2a and the outer peripheral surface to be the lower surface side 2b so that the side 2b becomes the outer peripheral surface i.e. an up-loop 23 by twisting the strip.