An input device comprises: an operation unit having a display part that can display a plurality of character keys and a touch screen; a subsequent input candidate search unit that, upon detection of contact to an area corresponding to a specific character key in the touch screen, searches among the input candidate string for an input candidate string with a first character being a specific character corresponding to the area thus detected, as a subsequent input candidate string; a subsequent input candidate determination unit that, with regard to the subsequent input candidate thus found, determines a subsequent character following the first character as a subsequent input candidate character; and a display exchange unit that displays a character key of the subsequent input candidate character thus determined, by exchanging with a character key arranged in the vicinity of a character key for the specific character as the first character.