PURPOSE: To shorten a required measuring time by the continuation of measuring operation and the simultaneous rotation of a polarizer and an analyser by continuously rotating the polarizer and the analyser at different speeds and calculating the respective azimuth angles of both of them at the point of time when a light detection signal becomes zero from the timewise change of the light detection signal at the time of rotation.

CONSTITUTION: A control unit 5 sets respective polarizing elements, that is, a polarizer 1, a 1/4 wavelength plate 2 and an analyser 3 to a reference azimuth and subsequently sets the 1/4 wavelength plate 2 to an azimuth of +45°. Thereafter, the polarizer 1 and the analyser 3 are rotated at predetermined angular velocities ω', ω and the output of a photodetector 4 is sampled at a definite time interval and stored in a memory along with the azimuth angles of the polarizer 1 and the analyser 3 at the time of rotation. Next, the azimuth angle of the wavelength plate 2 is set to -45° to repeat the same measuring operation. When the analyser 3 is rotated at a higher speed, the azimuth of the analyser 3 at the trough where the intensity of light is min. before and after a point of time (t) is a value near to an extinction angle and, when the polarizer is rotated at a higher speed, the extinction angle and azimuth of the polarizer 1 can be also calculated in the same way.