1. A character input device for a mobile terminal including a plurality of keys, each key being assigned characters of at least two different character types, wherein the mobile terminal is previously set in a character input mode of one of the at least two different character types, while characters of another of the at least two different character types can be selectively input, said character input device comprising: at least one temporary mode conversion key included in the mobile terminal; a memory for storing character codes of the at least two different character types assigned to each of the plurality of keys; and a controller for selecting one of the previously set character input mode and a temporary character mode, based on an input of the temporary mode conversion key, and reading out and recognizing character codes of the at least two different character types from the memory according to the plurality of keys input and the selected character input mode.