A laser scanning microscope provided with a laser light source, an illuminating optical system for condensing the light from the laser light source to form a light spot on a specimen, a scanning device for causing relative movement of the light spot with respect to the specimen, a photodetector for measuring the amount of light transmitted or reflected by the specimen, and an optical system with positive refraction power for guiding the light beam, transmitted or reflected by the specimen, to the light-receiving plane of the photodetector, wherein the photodetector is composed of a two-dimensional image sensor provided in a position displaced from the conjugate point of the light spot by such an amount that the light beam forms a far-field diffraction pattern of the specimen on the light-receiving plane of the photodetector.