1. A storage unit which comprises a segment buffer which temporarily stores write data written from a host computer to a disk medium and read data read from the disk medium to the host computer; a management section which manages data; and a segment management table for managing at every segment the data stored in the segment buffer and storing control information about each segment comprising: an access data feature value calculating section which calculates feature values of data accessed by a read command or write command which accesses to an address different from those of the data stored in the segment buffer, when such a command is issued by the host and a corresponding segment is newly created; and a purged-segment feature value looking up section which retains feature values of data contained in any segment purged by an LRU rule and checks whether a read command issued later by the host computer accesses to the same address: said access data feature value calculating section calculates feature values of data to be stored in any new segment generated and relationships between preceding and succeeding commands, said purged-segment feature value looking up section retains said feature values for a predetermined period after segment data is purged from the segment buffer by the LRU rule because of no available free space in the segment buffer and stores the feature values of said purged segment as feature values of a segment with a high hit probability after the purge when an address stored by said purged segment is read again, and then said management section does not purge a segment which is selected to be purged according to the LRU rule if it has feature values similar to those of the segment having a high hit probability after the purge, while purging segments with different feature values.