Abstract: A focus detecting optical system includes a reduction lens having a positive refractive power that is located between an objective lens and an image taking plane on which a primary image of a subject is formed by the objective lens, a separation plane mirror for separating a light passed through the reduction lens from an optical path toward the image taking plane, a pair of separator lenses for re-forming an image formed through the reduction lens and the separation plane mirror into a pair of secondary images, and a pair of image sensors for capturing the secondary images. The optical axis of the reduction lens is coaxial with the optical axis of the objective lens.