A photon counting device includes a plurality of pixels each including a photoelectric conversion element configured to convert input light to charge, and an amplifier configured to amplify the charge converted by the photoelectric conversion element and convert the charge to a voltage, an A/D converter configured to convert the voltage output from the amplifier of each of the plurality of pixels to a digital value and output the digital value, a correction unit configured to correct the digital value output from the A/D converter so that an influence of a variation in a gain and an offset value among the plurality of pixels is curbed, a calculation unit configured to output a summed value obtained by summing the corrected digital values corresponding to at least two pixels, and a conversion unit configured to convert the summed value output from the calculation unit to a number of photons.