The optical system for a microscope includes an illumination lens group G11 and an objective optical system. The objective optical system has an objective lens group G12, a first image forming lens group G13, and a second image forming lens group G14, which are disposed in this order from the object side, and satisfies the following conditional expressions. NAL×2≤NAO≤NAL×15 . . . (1); 0.01≤NAL≤0.1 . . . (2); and 25≤MO≤100 . . . (3), where NAL denotes the numerical aperture of the illumination lens group G11, NAO denotes the numerical aperture of the objective lens group G12, and MO denotes the image-formation magnification of the objective lens group G12.