A stereomicroscope has a rotary mirror to switch an optical path of a beam from an objective optical system between a normal optical path A and an image optical path B. Accordingly, a main operator D1 can keep his or her eyes on eyepieces 15 when switching an optical image through the normal optical path A and an electronic image through the image optical path B from one to another. A zooming optical system 10 is horizontally arranged in the normal optical path A and a high-magnification optical path C is formed on a horizontally outer side of the zooming optical system 10, to reduce a vertical dimension of a body of the microscope and secure a wide work space.