The integrity of components of an operating nuclear power plant (NPP) is usually monitored on a daily basis by operator patrols. Currently, there is a great need to replace such human patrol activities by systems of automated remote monitoring in order to reduce radiation exposure and the severe workload imposed on operators. From this perspective, and under the sponsorship of Ministry of International Trade and Industry (MITI), the authors embarked upon a R&D project in 1991 with the objective of developing an autonomous mobile patrol system for NPPs. The project is scheduled to be completed in 1996. This paper describes the concept of the autonomous mobile robot system, which comprises a hierarchical autonomous mobile control system, a localization system based on pattern matching between the data profile from a laser rangefinder and an environment map, a method of constructing environment maps from 3D-CAD of a NPP, and a cableless robot system.