A storing unit stores appearance information of the extracted word in correspondence with a specific record of the specific word in an index data when a specific word extracted from a target file is stored in a static dictionary that stores a plurality of words and a plurality of compressed codes corresponding respectively to the plurality of words, the appearance information including, existence information indicating whether the specific word exists in the target file or the appearance information indicating how many times the specific word has appeared in the target file. The storing unit stores adds a specific record of the specific word to the index data and stores the appearance information when the specific extracted word is not stored in the static dictionary and stored in a dynamic dictionary whose records are added in a course of compression.