To provide an illuminator for a microscope capable of easily switching critical illumination and Koehler illumination with improved uniformity of illumination than before in a simple composition.

An illuminator for a microscope has: a first illumination optical system 14 with a lens array optical system LA1 including a first and a second lens array surfaces LS1, LS2 provided to be capable of insertion or removal on an optical path where a light flux passes and formed by lining up a plurality of lens elements; an objective lens 19a radiating the light flux transmitted through the first illumination optical system 14 toward a sample SP; and a position determination mechanism 14a for determining the position of the lens array optical system LA1 so that an optical axes of respective lens elements are in parallel with an optical axis OA of the light flux while the lens array optical system LA1 is inserted into the optical path. In this case, the second lens array surface LS2 is positioned at a focal position on a rear side of the lens array optical system LA1 and the position of the second lens array surface LS2 is in a conjugate relationship with a focal point on a front side of a focusing optical system 19a in a state where the lens array optical system LA1 is inserted into a prescribed area.