PURPOSE: To detect a focal shift with high sensitivity by providing two photoelectric detecting means each having a non-photosensitive body part of the almost same shape as a spot optical image, on a part of the photodetecting surface by shifting them from the image forming surface separated by a half mirror.

CONSTITUTION: A luminous flux from a light source 1 is made parallel by a lens 2, reflected by a half mirror 3 and made to form an image on a data 5 by a lens 4. Its reflected light is divided into two by a half mirror 8 through the lenses 4, 7, provided onto glass plates 9a, 10a having circular light shielding parts 9b, 10b, and photoelectrically converted 9, 10. Glass plates 9, 10, for instance, are shifted by (d) forward and backward from surfaces FP1, FP2 on which the reflected light is to be focused. Output of the detectors 9, 10 are amplified by a circuit which is not shown in the figure and denoted as A and B, A+B and A-B are derived and a division (A-B)/(A+B) is executed, and the lens 4 is moved forward and backward by driving a motor 12 so that said value becomes minimum. In such a way, the focusing which is precise and has high sensitivity is executed. It is suitable for observing a pattern of a mask or a wafer.