An observation optical system is disclosed with which the Fresnel reflection at the surfaces of optical members can be reduced, the transmittance can be improved, and the surface reflection ghost that occur among a plurality of optical surfaces can be suppressed. The observation optical system of the present invention comprises a plurality of optical surfaces, and a fine periodic structure, with a period smaller than the wavelength of incident light, is provided at an effective optical region of at least one surface among the aforementioned plurality of optical surfaces.