PURPOSE: To make a high-precision measurement even when a body to be measured is a pattern formed on an optically irregular material, by irradiating the body to be measured with interference fringes which vary in spatial frequency continuously, and finding a Fourier spectrum from the quantity of the transmitted or reflected light.

CONSTITUTION: Interference fringes of two pieces of luminous flux are realized and luminous flux is split by the 1st half-mirror 13 into two, which are superposed by the 2nd half-mirror 17 to form the fringes. The spatial frequency of this interference light varies continuously by the rotation motion of a galvanomirror 16. This interference light moves by the vertical reciprocation of the reflecting surface of a reflecting mirror 14. This interference light is used to measure the size of Bragg stripes 26.