Here, a region of the concave portion 11 d corresponds to an exposure portion in the exposure process and becomes a groove (or a pit). [0129] Next, as shown in FIG. 12B, plating processing, etc. with nickel or other material is performed on the obtained mastering master disk to be a thickness of, for example, 290 ??m or so, so that a master stamper 12 is formed. [0130] An electric plating method f