[en] For decommissioning of the Fukushima Daiichi Nuclear Power Station (FDNPS), the primary containment vessel (PCV) internal survey for fuel debris spreading has been needed. Remotely operated robots have been used for the survey. Localization of the robots during their operation is necessary to adjust sensor positions. However, it is difficult to calculate the robot position because the survey target areas have an unknown environment. In this research, we proposed a localization method which combines a stereo camera-based method for absolute localization and a periodic structure tracking method for relative localization for application to the unknown environment. The position accuracy for the proposed method was verified in a mock-up test, and we confirmed that the position accuracy satisfied the required accuracy of 30 mm. (authors)