These variable-magnification observation optical systems are provided with an objective system, an erecting system, and an eyepiece system. In order from the object side, the objective system has a positive first lens group, a positive second lens group, and a negative third lens group. In order from the object side, the eyepiece system has a positive fourth lens group and a positive fifth lens group. The erecting system is located between the first and second lens groups. In order from the object side, the fifth lens group has the following, with air gaps in between: a negative meniscus lens element, the concave surface of which faces the object side; a negative meniscus lens element, the convex surface of which faces the object side; and a biconvex positive lens element.