A photodetector according to an embodiment includes: a photodetector element unit including a first cell array including a plurality of first cells arranged in an array and a second cell array including a plurality of second cells arranged in an array, each of the first and second cells including a photoelectric conversion element, the second cell array being arranged to be adjacent to the first cell array; a first pulse height analyzer unit analyzing a pulse height of an electrical signal outputted from the first cell array; a second pulse height analyzer unit analyzing a pulse height of an electrical signal outputted from the second cell array; and a signal processing unit determining non-uniformity of a distribution of photons entering the first and second cell arrays using an output signal of the first pulse height analyzer unit and an output signal of the second pulse height analyzer unit.