In this study an EGS4 simulation code was used to calculate real energy spectrum from measured ${gamma}$-ray energy spectrum obtained using 2-dimensional radiation monitoring system. As a result, the $39{ imes}39$ response matrix was calculated the energy range of 0.1 to 2 MeV which energy interval of 50 keV The real energy spectrum for Co-60 radioisotope was calculated using inverse of response matrix. It was confirmed that the calculated response matrix was useful to the analysis of the measured energy spectrum for the radiation monitoring system.