A technique to prevent a retrieving process of a conversion rule from taking a longer time is provided. Provided are a conversion table in which a predetermined number of entry regions capable of storing a mapping between first data and second data smaller in size than the first data are included, the predetermined number of entry regions are divided into a plurality of bank regions, and each of the plurality of bank regions includes entry regions smaller in number than the predetermined number, a determination unit configured to uniquely determine, among the plurality of bank regions, a bank region corresponding to the first data, and a processing unit configured to search entry regions of the determined bank region the predetermined number of times each or a smaller number of times than the predetermined number each, output, when the second data corresponding to the first data is stored, the second data, and when the second data corresponding to the first data is not stored, register the second data corresponding to the first data in an entry region in which another piece of second data is not stored and output the first data.