PURPOSE: To obtain rational priority information while using an information area of finite size by updating a priority level on such a condition that the sum of pieces of priority information on N pieces of the 2nd character strings corresponding to the specification of the 1st character string is constant.

CONSTITUTION: When a KANA (Japanese syllabary) is inputted from the 1st character string input part 1, a character string retrieval part 2 retrieves the KANA string in a character string storage part 3, and a KANJI (Chinese character) string with top priority information among N corresponding KANJI strings is selected and outputted to an output part 5. When the output KANJI string is not a desired string, a next-candidate request part 8 is operated and a next-candidate retrieval part 6 retrieves and outputs a KANJI string with the next priority information. When this is the desired KANJI string, the 2nd character string selection part 9 is operated and a priority information updating part 7 fetches the priority information on the outputted KANJI string out of a storage part 4 and adds N-1 to the priority information on the selected string while subtracting one from each of (N-1) KANJI strings on the condition that the priority information is smaller than a specific permissible value and the sum of pieces of priority information is constant; and the resulting strings are stored in the storage part 4.