1. A hierarchical storage device, comprising: a library storage device configured to store realtime stream data in units of segments subdividing each realtime stream data; a cache storage device configured to store selected segments among the segments stored in the library storage device; a memory unit configured to store random access point segment information from which a possibility for each segment to contain a point that can potentially be random accessed in future can be estimated; and a control unit configured to control a selection of the selected segments to be stored in the cache storage device according to the random access point segment information stored in the memory unit.