An optical device includes: a substrate a metal nanostructure formed on the substrate and containing metal particles and an organic molecular film formed on the metal nanostructure. A particle size of each metal particle is 1 nm to 500 nm when seen in a plan view. The organic molecular film includes holes penetrating in a thickness direction thereof. The holes are arranged in a two dimensional array along the surfaces of the metal particles. A size α of each hole satisfies 0.5 nm≦α≦5 nm, a period P of the holes satisfies P≦10 nm, and a thickness t of the organic molecular film satisfies t≦1 nm.
