An observation field of view of a microscope can be moved without moving or changing an objective lens and without varying position or state of a sample. A microscope optical system has a mirror that changes the direction of the optical path by reflection and is located in the optical path between an objective lens of the microscope and an image to be observed. The mirror is able to be tilted by changing the position of a reflecting surface of the mirror. Accordingly, the observation field of view is moved by tilting the mirror. In other words, the observation field of view can be moved without changing a positional relationship between the objective lens of the microscope and the sample.