PURPOSE: To determine the size of an object to be measured at a high speed, by calculating beforehand the correlation between the shadow of a known reference pattern and the objects to be measured of various sizes and comparing the numerical values and the results of measurement.

CONSTITUTION: The pattern of a black stripe 32 has the pattern of the apertures of a shadow mask 15 as a reference pattern. When R, i.e., the ratio between the max. value and min value of the quantity of the incident light to a detector 18 is determined, the width B of the black stripe 32 is determined by using a precalculated R-B curve. A signal processing unit 21 consists basically of a CPU and an ROM. The CPU has the functions of changing the angle of a galvanomirror 13, assigning the address of the ROM by using the ratio R between the max. value and min. value in the section where the angle changes, after the change in the angle attains 1/3 of the aperture pitch (p) of the mask 15, fetching the data stored thereon and transmitting the same to a display device 22 and the function of controlling a moving device 17. The R-B curve is stored in the ROM. When the angle of the mirror 13 is known, the incident angle of the parallel luminous fluxes is assigned and to which stripe 32 the fluxes correspond is known.