An observation optical system includes, in order from a sample side, an infinity-corrected objective, a first lens group having a positive power, a second lens group having a positive power, and a tube lens having a positive power. An intermediate image is formed between the first lens group and the second lens group, and at a front side focal position of the second lens group. A space along an optical axis between the objective and the first lens group is configured to be changeable, and a space along an optical axis between the second lens group and the tube lens is configured to be changeable.