An optical system for observing front and lateral objects includes a negative front group having a reflective refractive element, an aperture stop, and a positive rear group having a moving lens unit that is movable along the optical axis. The reflective refractive element includes a front-object-side first face having a transmitting surface and a reflective surface annularly formed around the transmitting surface, an image-side second face having a transmitting surface and a reflective surface annularly formed around the transmitting surface, and a face formed as a transmitting surface between the first and second faces. The optical system performs focusing onto an object while hardly changing, in the entire observation area, an area in which an image of the front object is formed, by moving the moving lens unit to the extent that the refractive powers of the front and rear groups hardly change relative to each other.