PURPOSE: To apply this invention to sheets of various types by controlling the minimum outside radius of a roller to specified dimensions, coating the roller surface with a sleeve of a flexible material, applying liq. pressure and changing the outside curvature of the sleeve.

CONSTITUTION: The minimum outside radius Rm of a medium-high roller is controlled to ≥E1/2σ0, where E denotes the Young's modulus of the material of the sheet, (e) denotes the thickness of the sheet, and '0 denotes the yield strength of the material. As to the roller 14B, a solid, cylindrical core 20 is coated with a flexible sleeve 22 to form a chamber 28. The chamber 28 is applied with liq. pressure to form the shape of the sleeve 22 to the circular-arc shape. The radius Rt of the circular-arc is controlled to ≤E1/2σ0. The deformation in the longitudinal direction of the sheet generated by the medium-high roller 14B lies in an elastic range and does not lie in a plastic range.