To provide a compact ocular optical system whose constitution is simple, which is easily manufactured and by which a clear observed image is obtained.

This ocular optical system is provided to observe an object (3) as a virtual image without forming the intermediate image of the object (3). A lens component (11), a plane parallel plate type diffraction optical device (12) and a reflection mirror (13) are arranged in an optical path leading to an observing eye (5a) from the object (3). The optical device (12) has a plane parallel plate where a diffraction optical surface is formed only on an either surface.