Patent US 9,720,976

An extracting method includes storing to a storage device: files that include character units; first index information indicating which file includes at least one character unit in a character unit group having a usage frequency less than a predetermined frequency and among character units having common information in a predetermined portion, the usage frequency indicating the extent of files having a given character unit; second index information indicating which file includes a first character unit having a usage frequency at least equal to the predetermined frequency and among the character units having common information in a predetermined portion; and referring to the first and second index information to extract a file having character units in the first and second index information, when a request is received for extraction of a file having the first character unit and a second character unit that is included in the character unit group.