The present invention has for its object to provide an optical device whose size is made small. In order to achieve this, according to the present invention there is provided an optical device in which a clad and a core whose refractive index is relatively higher than that of the clad are formed on a substrate and light can be transmitted through the core, wherein the core is provided with an input light transmitting pattern section, a diffraction pattern section, and a phase difference generating pattern section. Further, at least in the phase difference generating pattern section, a part, where refractive indexes are different respectively in the direction parallel to the substrate and in the direction perpendicular to the substrate, is provided.