PURPOSE: To calculate the phase difference of a transparent film by emitting a laser beam to the transparent film to be measured formed on the reflecting surface of a base through a polarizer, a modulator, and a first optical element, reflecting the reflected light emitted from the transparent film in different right-angled directions by first and second optical elements, respectively, and detecting the light through an analyzer by a light detector.

CONSTITUTION: A transparent film 20 to be measured is formed on the reflecting surface 21a of a base 21. The output light of a laser 11 is emitted to the transparent film 20 through a polarizer, a photoelastic modulator 13, and a first half mirror prism 14. This light is reflected by the reflecting surface 21a, advanced in an optical path A1, and reflected to a right-angled optical path A2 by the reflecting surface 14b of the prism 14. This light is collided with the reflecting surface 15b of a second half mirror prism 15, and reflected in the direction right-angled to the optical paths A1, A2. Thereafter, the light is emitted to a light detector 17 through an analyzer 16. The intensity of the light passed through the analyzer 16 is measured by the light detector 17, the measurement value of the detector 17 is processed by an arithmetic device, and the phase difference of the transparent film 20 can be highly precisely measured.