Abstract: A Japanese Hiragana input character string of “a-i” is displayed while a single Japanese Hiragana character of “ti” is displayed as a display character for representing a “ta” row key assigned with a “ta” row character group of “ta”, “ti”, “tu”, “te”, and “to”. When a user then appoints the “ta” row key, “ti” is added to the tail end of the input character string to newly form the input character string of “a-i-ti” containing three characters. When each of “ka” and “ki” does not effectively follow “a-i-ti” in tree structure index data, a display character for a “ka” row key assigned with “ka”, “ki”, “ku”, “ke”, “ko” is switched into “ku” while skipping “ka” and “ki”. Further, when “tu” does not effectively follow “a-i-ti” in the index data, a display character for the “ta” row key is switched from “ti” into “te” while skipping “tu”.