Radial measurements of bremsstrahlung photons show high-energy intensity beyond a critical energy from electron cyclotron resonance (ECR) heating and its nature is not well understood so far. For the first time we have measured the bremsstrahlung photons energy intensity from 28-GHz ECR ion source at Busan Center of KBSI. Three round type NaI(Tl) detectors were used to measure the bremsstrahlung photons emitted at the center of the ECRIS at the same time. Another NaI(Tl) detector was placed downstream from the ECR ion source for monitoring photon intensity. The ECR ion source was operated at Radiofrequency (RF) power of 1 kW to extract 16O beam with a dominant fraction of O3+. Bremsstrahlung photons energy spectra were measured at the center of the ECR ion source. We studied possible systematic uncertainties from different characteristics among the three NaI(Tl) detectors by repeating measurements alternatively. Geant4 simulation was performed to take the geometrical acceptance and energy-dependent detection efficiency into account due to large non-uniformity in the material budget. We extracted true bremsstrahlung energy spectra from the 28-GHz ECR ion source using the inverse-matrix unfolding method. The unfolding method was based on a full geometry Geant4 model of the ECR ion source. The high energy intensities of the bremsstrahlung photons at the center of the ECRIS were explained by the internal structure and shape of ECR plasma.