For taking out the melted down nuclear fuel debris in Fukushima nuclear power plant, localization of the debris inside plant is required. Because of high radioactive in Fukushima nuclear power plant, a mobile robot mounting the Compton camera is used in order to investigate the power plant instead of human. However, the mobile robot has uncertainties of its pose. We propose a method to localize radiation source and the robot using extended Kalman Filter (EKF)- based simultaneous localization and mapping (SLAM) with Compton camera. We evaluated the performance of proposed method in a simulated environment.