The ratio of the difference between a surface distance L at each of the different positions in a width direction DW of a metal sheet and a minimum surface distance Lm to the minimum surface distance Lm is an elongation difference ratio. The elongation difference ratio in a center section in the width direction DW of the metal sheet is less than or equal to 3×10−5. The elongation difference ratios in two edge sections in the width direction DW of the metal sheet are less than or equal to 15×10−5. The elongation difference ratio in at least one of the two edge sections in the width direction DW of the metal sheet is less than the elongation difference ratio in the center section in the width direction of the metal sheet.