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 stored in the library storage device 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, the control unit including a discarding segment selection unit configured to select segments that are candidates for discarding from the cache storage device, among the selected segments stored in the cache storage device, according to the random access point segment information stored in the memory unit, wherein the control unit includes an update unit configured to update the random access point segment information for a segment containing a certain point on the realtime stream data upon receiving a seek request with respect to said certain point.