The storage and later retrieval of observed data is an essential part of any support system aiding the safe operation and maintenance of a nuclear power plant. Archiving such data, however, which is broadly spread over space and time, is challenging: problems include how to reduce the amount of data, how to maintain relationships between data, how to normalize the data and so on. We propose a new concept, 4D Visible Memory, whose basic idea is to store the data in a virtual space extended in the time dimension, as a solution. In this paper, a specific system which embodies the concept in the special case of archiving the image data gathered by mobile robots is introduced. Some experimental results show the actual usefulness of the concept.