For safety and certain removal of the melted down nuclear fuel debris, the localization of radiation sources should be required. In this paper, we propose an approach to estimate the accurate localization of radiation sources by utilizing a gamma-ray CT method with a detector mounted on a mobile robot that has a pose uncertainty. Gamma-ray CT method aggregates measurement data obtained from the detector to localize radiation sources. For applying gamma-ray CT method, detector’s position that the measurement data obtained is important. In a simulation experiment, we confirmed the detector’s position errors and correlated them with the accuracy of the localization of radiation sources.