Compiler device for optimizing program which manipulates a character string includes append instruction detection unit, store code generation unit, and append code generation unit. The append instruction detection unit detects an append instruction which appends a character string to a string variable for storing a character string, in the program. The store code generation unit generates, a substitute for each of a plurality of the append instructions detected by the append instruction detection unit, a store code for storing data of an appendant character string to be appended to the string variable by the append instruction into a buffer. The append instructions append the character strings to the same string variable. The append code generation unit generates append code for appending a plurality of the appendant character strings to the string variable, at a position executed before an instruction to refer to the string variable in the program.