The invention relates to an optical device which can increase the spread of a beam diameter in the depthwise direction by a simple configuration in comparison with that by prior art devices. The optical device includes a substrate, an optical path formed on the substrate, and a diffraction propagation region, provided between the optical path and an end face of the substrate, for propagating light emitted from the optical path with diffraction. The diffraction propagation region includes a first groove, formed therein, adapted to block part of components of the propagated light in a depthwise direction of the substrate.