An observation optical system unit includes a first lens 58a which is positioned closest to an object side, among rear-group lenses 58, and which is configured by a stepped lens including a proximal portion 58b, an outer circumference of which is held by a rear-group lens frame 57, and a protruding portion 58c formed to have an outer circumference smaller than the outer circumference of the proximal portion 58b and protruding into a region At where a moving lens frame 65 is allowed to advance and retract, where the moving lens frame 65 overlaps with the protruding portion 58c at a retracted position.
