6. The method according to claim 5 , wherein the at least one apparatus further performs: receiving an input of a search character group; identifying an object file that includes the search character group by referring to the first index information; retrieving information related to the search character group from the identified object file; and outputting an result of the retrieving.
5. A method, comprising: performing, by at least one apparatus: obtaining a word appearance frequency information related to an appearance frequency of each word belonging to a basic word group, and a single character appearance frequency information related to an appearance frequency of each single character belonging to a designated single character group, wherein the appearance frequency of each word or each single character indicates an appearance frequency thereof in an object file group; extracting a plurality of specific words from the basic word group based on the word appearance frequency information; extracting a plurality of specific single characters from the designated single character group based on the single character appearance frequency information; dividing a character code of each single character that is included in the designated single character group but not included in the specific single characters, into upper bits of a divided character code and lower bits of the divided character code; and generating for the object file group: first index information indicating for each file the presence/absence of each specific word, second index information indicating for each file the presence/absence of each specific single character, third index information indicating for each file the presence/absence of each single character including the upper bits of the divided character code, and fourth index information indicating for each file the presence/absence of each single character including the lower bits of the divided character code.