PURPOSE: To realize a measuring method in which the quantity of shear and a difference in optical path length due to vibration and the disorder of air are hardly generated by separating measurement light into two pieces of luminous flux and shifting them laterally, and scanning interference fringes for one period by a Bavinet-Soleil compensator.

CONSTITUTION: The Bavinet-Soleil compensator 1 and a birefringent polarizer 2 such as a Wollaston prism are arranged on the optical path of the measurement light which contains information on an object of measurement as a wave front shape. The measurement light is separated by the polarizer 2 into two pieces of luminous flux, which are shifted from each other. Further, the compensator 1 scans interference fringes for one period. Thus, the measured value is transmitted through the compensator 1 and polarizer 2 and sheared by the polarizer 2, and the compensator 1 makes the fringe scan, so the optical path of measurement is formed as a common path and variation in shear quantity and optical path length difference due to vibration and the disorder of air is hardly caused.