In this paper, we present a new method to protect soft-ware against illegal acts of hacking. The key idea is to add a mechanism of self-modifying codes to the original program, so that the original program becomes hard to be analyzed. In the binary program obtained by the proposed method, the original code fragments we want to protect are camouflaged by dummy instructions. Then, the binary program autonomously restores the original code fragments within a certain period of execution, by replacing the dummy instructions with the original ones. Since the dummy instructions are completely different from the original ones, code hacking fails if the dummy instructions are read as they are. Moreover, the dummy instructions are scattered over the program, therefore, they are hard to be identified. As a result, the proposed method helps to construct highly invulnerable software without special hardware.