1. A method of optimizing character string output processing, comprising: acquiring an output program for sequentially executing a plurality of character string output instructions, and for outputting a text in which a plurality of output character strings are combined; converting the character code set of the character string constants, which is outputted by at least one of the character string output instructions, from a first code set for internal processing to a second code set for output, before the output program is executed; determining, for each character string output instruction for outputting a character string constant, whether or not a character code set of a character string to be outputted has been previously converted into the second code set before the output program is executed; buffering the values of two or more character string variables to be outputted by two or more of the character string output instructions without converting the character code set from the first code set, during the execution of the output program; converting collectively the character code set of the values of the two or more buffered character string variables, from the first code set to the second code set; and outputting the character string constant whose character code set has been converted, and the values of the two or more character string variables whose character code set has been converted, as at least a part of the text.