A photo detector having an electrically conductive thin film and a light-receiving unit. A coupling periodic structure is provided on a surface of the film and converts incidence light to surface plasmon. The coupling periodic structure has an opening that penetrates the obverse and reverse surfaces of the thin film. The light-receiving unit is provided at one end of the opening in the surface that is opposite to the surface on which the coupling periodic structure is provided. The opening is shaped like a slit and is broader than half (½) the wavelength of the surface plasmon in a direction that intersects at right angles with a polarization direction of the incidence light and is narrower than half (½) the wavelength of the surface plasmon in a direction parallel to the polarization direction.