To provide a method of detect a relative positional shift between periodic patterns on the obverse side and a reverse side, without cutting a transparent sheet.

In this method of measuring the relative positional shift between the periodic patterns formed respectively in the obverse side and the reverse side of the transparent sheet 2 and continued unidirectionally, an area 24 formed with the periodic pattern 21 only in the obverse side, and an area 25 formed with the periodic pattern 22 only in the reverse side are provided in the transparent sheet, the two areas are irradiated with light to detect the transmitted light, so as to detect positions X3, X5, X7 of characteristic points f2, f3, f4 of the periodic pattern in the obverse side based on the light transmitted through the area 24, and positions X2, X4, X6 of characteristic points r2, r3, r4 of the periodic pattern in the reverse side based on the light transmitted through the area 25, and the positions of characteristic points of the periodic pattern on the obverse side is compared with those of the reverse side to calculate the positional shift ΔX between the periodic patterns on the obverse and reverse sides.