Abstract: An optical system to be inspected is scanned with slit-shaped illuminating beams. The beams from the optical system are received with two or more pairs of photoelectric converting elements and the refractive power of the above mentioned optical system is measured on the basis of the phase difference of the output signals between the photoelectric converting elements forming the respective pairs. The angle of inclination of the slit-shaped illuminating beams to the scanning direction is made in two or more different known angles. An output device omitting the discriminating signal of the angle of inclination of the illuminating beams is provided and the discriminating signal and phase difference inputted to an operating circuit to obtain a refractive power.