Abstract: Light emitted from a light source (1) reaches a sample surface (2) via an objective lens (6). An edge (7a) of a louver (7) is located at a a position which is conjugate with a predetermined reference position across the objective lens. When the sample surface is displaced from the reference position, light reflected from the sample surface impinges upon the louver is reflected therefrom and is then detected by a photo-sensor (8). When the surface coincides with the reference position, the light does not reach the photo-sensor. Thus, a displacement of the sample surface from the reference point can be detected by monitoring output signals from the photo-sensor.