1. A method for realizing a compiler device for optimizing a conversion of a character coding system of a character stored in a string variable in a target program to be optimized by a computer, comprising: a conversion instruction generation step for generating a conversion instruction, to convert characters written by a first character coding system into a second character coding system and to store the converted characters in a string variable, said conversion instruction being generated at a position to be executed prior to each of a plurality of procedures by which the characters are read out and used in the second character coding system, and being generated due to a CPU operation of the computer; and a conversion instruction removal step, where with respect to each of conversion instructions generated by the conversion instruction generation section, the conversion instruction is removed from the target program due to the CPU operation of the computer if a character in the second character coding system is stored in the string variable in each of execution paths executed prior to the conversion instruction.