A biological detecting system comprising a light source; a condensing optical system for condensing a light beam from the light source and irradiating the surface of a sample to be detected with a spot of light, an imaging optical system for condensing light reflected or scattered by an irradiated portion of the sample and forming an image of the irradiated portion at a predetermined location, and a photodetector arranged at the predetermined location to detect the size of the image of the irradiated portion and output a detection signal indicating the detected size of the image.