To provide an optical system achieving rapid observation by a confocal optical system.

The optical system 1 includes: the confocal optical system 3 to radiate illuminating light emitted from an illuminating light source 21 to a specimen 8 through a confocal minute aperture 34a so as to obtain passing light reflected by the specimen and passing through the confocal minute aperture 34a; a photodetector part 41 to detect the passing light; a displacement part 5 to displace the focal position of the confocal optical system to the specimen in an optical axis direction; and a processing part 60 to read and process detection data from the photodetector part 41 at a plurality of positions obtained by displacing the focal position. The optical system 1 is constituted so that the processing part may adjust the intensity of the light radiated to the specimen 8 based on the detection data read at the plurality of positions.