An apparatus for distinguishing an energy band of a photon in a readout circuit that counts photons in multi-energy radiation incident onto a sensor for each energy band includes an integrator configured to accumulate an electric signal received from the sensor that has undergone photoelectric conversion from the photon a comparator configured to compare an accumulated electric signal received from the integrator with one of a plurality of threshold values and a signal processor configured to instruct sequential switching from one of the plurality of threshold values to another one of the plurality of threshold values according to a result of a comparison received from the comparator and output a digital signal that distinguishes an energy band of the photon based on results received from the comparator of sequential comparisons of the accumulated electric signal with the plurality of threshold values.
