To make it possible to cover the shortest observation distance without requiring a close-up lens by providing a telescopic optical system which has, successively from an object side, an objective optical system having a stationary first lens group and a movable second lens group, an erecting optical system and an ocular optical system and satisfying specific conditions.

This telescopic optical system consists of, successively from the object side, the objective optical system 10 having the stationary first lens group G1 and the movable second lens group G2 for focusing, an erecting optical system 20 and an ocular optical system 30. The object image by the objective optical system 10 is formed on a primary imaging surface 10P and the image is observed through the ocular optical system 30. The condition equations (1) d/f<0.4, (2) f/f1>d/f-0.3, (3) f/f1<1.4-d/f are satisfied in the telephoto optical system. In the equations, d: the distance from the rear side principal point of the first lens group G1 in the infinite observation state to the front side principal point of the second lens group G2, f: the combined focal length of the objective lens system in the infinite observation state, f1: the focal length of the first lens group G1.