PURPOSE: To realize continuous variation of retardation of a large, uniform plane easily, by plying two cellophane sheets equal in double refraction in a way that the X-axis of one sheet be parallel to the Z-axis of the other sheet, and rotating one sheet around the X-axis.

CONSTITUTION: Cellophane sheets 1, 2 having equal double refraction are stretched by holders 3, 4 respectively, and are overlapped so that the X-axis of one sheet by parallel to the Z-axis of the other sheet. When the cellophane 2 is rotated about a rotary shaft 5, the retardation given to the light transmitted in the viewing direction increases continuously along with the angle of rotation, and the combined retardation increases continuously from zero. When cellophane materials of which retardation is about 900 nm are used, a photoelastic strain standard element capable of varying the combined retardation in the range of 0 to 300 nm may be obtained.