1. A character input apparatus comprising: a completing unit configured to determine, based on a character string formed by at least one character, which is input by a user, a character string formed by at least one character which is to be input as a portion following the character string input by the user, and complete the character string input by the user, by adding the determined character string as a portion complementary thereto; a voice guidance setting unit configured to set to perform voice guidance for reading aloud a character which has been input by the user; and a control unit configured to control the completing unit to perform completion of the input character string if the voice guidance setting unit does not set to perform the voice guidance, and to control the completing unit not to perform completion of the input character string if the voice guidance setting unit sets to perform the voice guidance.