The present technology relates to an electromagnetic wave processing device that enables reduction of color mixture. Provided are a photoelectric conversion element formed in a silicon substrate, a narrow band filter stacked on a light incident surface side of the photoelectric conversion element and configured to transmit an electromagnetic wave having a desired wavelength, and interlayer films respectively formed above and below the narrow band filter, and the photoelectric conversion element is formed at a depth from an interface of the silicon substrate, the depth where a transmission wavelength of the narrow band filter is most absorbed. The depth of the photoelectric conversion element from the silicon substrate becomes deeper as the transmission wavelength of the narrow band filter is longer. The present technology can be applied to an imaging element or a sensor using a plasmon filter or a Fabry-Perot interferometer.