An atomic force microscope comprises a probe having a sharply-pointed tip end. The probe is supported on the free end portion of a cantilever and is close to the surface of a specimen. When an interatomic force is produced, the cantilever is deformed, and the probe is displaced. The displacement of the probe is detected by an optical system. A light beam emitted from a light source is collimated by a lens, and reflected by a polarized beam-splitter, and also by a half-mirror. Then, the light beam passes through a quarter wavelength plate and an objective lens, such that the light is converged on the cantilever. The reflected light beam from the cantilever returns along the same optical path and passes through the splitter. The light beam is divided into two light beams at the splitter. These two light beams are reflected by respective prisms and are then incident on respective photodetectors. These photodetectors detect the displacement of the probe.