A light diffraction element, that has cells, includes first regions and second regions. Each of the cells comprises one of the first regions and one of the second regions. Each of the first regions has a thickness or a refractive index that is independently set. The second regions have a uniform thickness or a uniform refractive index. The first regions allow first polarized components of signal light to pass through. The second regions allow second polarized components of signal light to pass through. The second polarized components are different, in polarization direction, from the first polarized components. The light diffraction element performs optical computing by causing the first polarized components of signal light that have passed through the first regions to interfere with each other. The first polarized components of signal light output from the light diffraction element indicate information after the optical computing.