An observation optical system of a real-image type includes, in order from the object side, an objective system, a reverse-erecting system that s erects an inverted image formed by the objective system, and an eyepiece system that allows a pupil to observe an erect image formed by the reverse-erecting system. The objective system includes, in order from the object side, a first lens having a negative power and a second lens having a positive power. The eyepiece system includes, in the order from the object side, a third lens having a positive power, a fourth lens having a negative power, a fifth lens having a positive power, and a sixth lens having a positive power.
