Provided is an infrared light detector 100 wherein a light coupling mechanism 110 is configured by a metal thin film or metal thin plate in which a plurality of windows apart from each other are formed. Each of the windows is formed by multangular shapes in which a part of the internal angles are obtuse angles. The plurality of windows are periodically arrayed in postures having translational symmetry in at least two directions. The array cycle p of the plurality of windows are set according to a wavelength λ' of the infrared light of a substrate including a first electronic layer 110 so as to fall within a range with reference to a value at which a perpendicular oscillating electric field component in a first electronic region 10 indicates a peak value.