To provide an optical system capable of switching between forward-view observation and simultaneous observation to simultaneously observe the forward view and all the side-views, and also, capable of minutely observing the observation image in all side-directions in the view field in the simultaneous observation state to simultaneously observe the forward view and all the side-views.

The optical system includes: a forward view observation optical system 1; an all side-views observation optical system 2; a variable diaphragm 3 disposed on the image side of the observation optical system 2; an imaging optical system 4; and an image sensor 5. The variable diaphragm 3 is made switchable between a first observation view field including only the forward view field and a second observation view field including a forward view field whose angle is narrower than that of the first observation view field and all the side-view field. The observation optical system 1 is disposed on the object side of the observation optical system 2 while facing the aperture 2a. When being switched to the first observation view field, the variable diaphragm 3 shields the optical path of the observation optical system 2, and when being switched to the second observation view field, the variable diaphragm 3 opens the optical path of the observation optical system 2.