The present invention enables switching between wide-field observation and high-magnification observation during in vivo examination of a small laboratory animal or the like using a narrow-diameter objective lens, without changing the objective lens. There is provided a microscope optical system including an objective lens for collecting light from an examination target; an image-forming lens for imaging the light collected by the objective lens onto a detection device; and an auxiliary optical system, having positive power, which is provided so as to be capable of being inserted in and removed from a light path between the objective lens and the image-forming lens.