A character string processing apparatus converting a character string encoded by a first encoding method to a second encoding method selected from a plurality of encoding methods is disclosed. The character string processing apparatus includes an encoding method determination part that selects the encoding methods, obtains, with respect to each selected encoding method, at least one of the number information and the position information of one or more replacement codes at the time of converting the character string to the selected encoding method, and determines the second encoding method based on at least one of the number information and the position information.