The present optical system comprises a forward observation optical system, an entire lateral circumference-observation optical system, a variable stop arranged on the image side of the observation optical system, an image-forming optical system, and an image pickup element, and the variable stop is formed so as to be capable of switching an observation field of view at least to a first observation field of view including only a forward field of view and to a second observation field of view including a forward field of view with a narrower angle than that of the first observation field of view and a field of view of the entire lateral circumference. The observation optical system is arranged on the object side of the opening in the central position of the observation optical system. The variable stop intercepts an optical path for the observation optical system when an observation field of view is switched to the first observation field of view, and opens the optical path for the observation optical system when an observation field of view is switched to the second observation field of view.
