PURPOSE: To make a keyboard compact by reducing the number of keys without sharply increasing the number of key strokes.

CONSTITUTION: Each kind of character can be predictably displayed related with various keys 24 in each kind of prescribed character set arrange based on the character preceding to an inserting point in a corresponding text field on a display, or the character inputted at last from a keyboard 10. Each key 24 of the keyboard 10 can display many different characters, and each key displays only one character at one time, and each character is displayed at only one key place. A user can manually change the character set arrangement. When the user selects a specific key, and selects the inserting point in the text field, all the keys 24 are updated in order to display the character set arrangement corresponding to the character set including the character having the highest possibility of being selected by the user from the next time based on the frequency of the combination of the specific characters used in specific language or specific application.