PURPOSE: To enable smooth document input by automatically changing an input mode and an input shift which are currently set into an input mode and an input shift to which a user to change.

CONSTITUTION: An input mode control part 3 interprets a character key string, inputted from an input part 1 through a control part 2 in the currently set input mode and a KANA(Japanese syllabary) KANJI(Chinese character) conversion part 6 converts the interpretation result from KANA to KANJI; if the result obtained by evaluating the conversion result by a KANA-KANJI conversion evaluation part 7 is not reliable enough, the input mode control part 3 temporarily switches the device to another input mode to interpret the inputted character key string, which is converted from KANA to KANJI. When the conversion result is reliable enough, the KANA-KANJI conversion part 6 outputs the conversion result to an output part 10. Once the user determines the outputted conversion result, the input mode control part 3 switches the device to the different mode regularly. Consequently, the improper input mode is automatically changed to the input mode that the user intends and smooth document input can be performed.