An opening radial dimension is set as a radial direction permissible dimension K of the metal coil as expressed by the following equation or lower. $K=\frac{\mathrm{Yp}Z}{2\mathrm{EI}}{R}^2\left(-\sin \cos \right)$ Wherein: K is a radial direction permissible dimension at the load action point position in mm; Yp is a yield stress of the metal sheet, in kgf/mm.sup.2; Z is a section modulus of the metal sheet, in mm.sup.3; R=(a metal coil radius r)(the plate thickness t of the metal sheet), in mm; E is a Young's modulus of the metal sheet, in kgf/mm.sup.2; I is a second moment of area of the metal sheet in mm.sup.4; and is an angle in radians about the axis of the metal coil from the load action point to the nearest restraining roll along a rewind direction of the metal coil.