An observation optical system unit includes a first lens which is positioned closest to an object side, among rear-group lenses, and which is configured by a stepped lens including a proximal portion, an outer circumference of which is held by a rear-group lens frame, and a protruding portion formed to have an outer circumference smaller than the outer circumference of the proximal portion and protruding into a region where a moving lens frame is allowed to advance and retract, where the moving lens frame overlaps with the protruding portion at a retracted position.
