An ambiguous key character input method is adopted, enabling appropriate character input without enlarging or complicating the structure of a portable terminal. In a mobile telephone having a keypad where a plurality of characters are respectively allocated to input keys according to number keys from 0 to 9, like a telephone keypad, am ambiguous key input method is adopted where a user respectively presses keys containing each character of a character string or word they want to input one at a time. At the mobile telephone, a disambiguation server network connected using a wired network or a wireless network returns a character string or selection list postulated by a user intending input from a series of keys received from the portable terminal to the portable terminal, and by selecting a character string or word the user wants to input from the character string selection list the character string or word is efficiently input.