Optical system and optical microscope

An optical system which corrects an intensity distribution of incident light to a flat intensity distribution includes: a first lens group which includes at least one lens and has a positive refracting power; a second lens group which includes at least one lens and has a negative refracting power, the second lens group being positioned behind the first lens group in a direction of the incident light; and a third lens group which includes at least one lens and has a positive refracting power, the third lens group being positioned behind the second lens group in the direction of the incident light. In the optical system, the incident light is collimated, and the intensity distribution of the incident light is corrected to the flat intensity distribution by spherical aberrations of the first lens group, the second lens group and the third lens group.