To obtain a Keplerian zoom finder optical system where chromatic aberration is satisfactorily corrected all over the zooming area by providing a diffraction optical surface on the 3rd group of a zoom lens as an objective, lens including a 1st positive group, a 2nd negative group and the 3rd positive group in order from an object side.

The objective lens tg is the zoom lens constituted of the 1st positive group g1, the 2nd negative group g2 and the 3rd positive group g3 arranged in order from the object side. As for the objective lens tg constituted of three group zoom lens, that is, positive, negative and positive groups, the diffraction optical surface(DOE) is installed on the 3rd group g3. Thus, the chromatic aberration is satisfactorily corrected all over the zooming area with a small number of constitutional lenses. Besides, the zooming operation is performed by moving the 2nd group g2 and the 3rd group g3, the fluctuation of the chromatic aberration is effectively reduced all over the zooming area, and also, the number of constitutional lenses is effectively reduced.