PURPOSE: To measure the retardation of a sample readily in a short time by measuring the distance FSR between the peaks of the transmittances of the same polarized light and the distance δ between the peaks of the transmittances of the different polarized lights, and obtaining the retardation of the sample based on the result of the comutation of πδ/FSR.

CONSTITUTION: In this convention, there are following devices. A resonator RS comprises a pair of facing mirrors 4 and 6 having the high reflectivities. Variable means 6,10,11 and 12 can change the length of the optical path between a pair of the mirrors 4 and 6 of the resonator RS. A single-model laser 1 which emits input laser light is provided at one end of the optical axis of the resonator RS. A photodetector 9 detects the output laser light from the other end of the resonator. The distance FSR between the peaks of the transmittances of the same polarized light when a sample 5 is provided at the optical path of the resonator RS and the distance δ between the peaks of the transmittances of the different polarized lights are measured. The retardation of the sample is obtained based on the result of the computation of πδ/FSR.