PURPOSE: To accurately measure double refraction at a specific position on a disk substrate in its rotation by rotating the disk substrate and measuring the double refraction index at the time of the rotation of the disk substrate.

CONSTITUTION: Light emitted by a laser light source 8 is polarized circularly and made incident on the disk substrate 7. The disk substrate 7, on the other, is fitted on a spindle motor 6, which is brought PLL control with rotating speed control pulses generated by dividing the frequency of pulses from a reference clock generator 1 by a frequency divider 2. The luminous flux reflected by the disk substrate 7 is guided to a rotary analyzer 13. The rotary analyzer 13 is driven by a motor brought under PLL control with rotating speed control pulses generated by dividing the frequency of the pulses from the reference clock generator 1 by a frequency divider 3. The output of a photodetector 15 is inputted to a sampling hold circuit 17. This holding is timed at the constant position on the disk substrate 7 and a signal processing circuit 18 calculates the double refraction index.