A discrimination parameter calculation method for photon detectors in this invention, applies two types of fitting functions which approximate waveforms of count numbers relative to energy ratios to the data having accumulated output waveform signals, and calculates fitting parameters of both the fitting functions. Based on both the fitting parameters, count numbers which are 1/n of peaks of both the fitting functions, and a value of the energy ratio of both the fitting functions corresponding to the count numbers is calculated as discrimination parameter k. Thus, whatever kind first photon detecting elements 35 and second photon detecting elements 37 may be, since discrimination parameter k is automatically calculated based on the fitting parameters after carrying out a fitting with the fitting functions, the discrimination parameter can be calculated with high accuracy. It is therefore possible to obtain relatively easily discrimination parameter k which can fully bring out performance of the photon detectors 17.