Existing researches suggest that the code clone (duplicated code) is one of the factors that degrades the design and the structure of software and lowers the software quality such as readability and changeability. However, the influence of code clones on software quality has not been quantitatively clarified yet.

In this paper, we have tried to quantitatively clarify the relation between code clones and the software reliability and maintainability of twenty years old software. As a result, we found that modules having code clones (clone-included modules) are more reliable than modules having no code clone (non-clone modules) on average. Nevertheless, the modules having very large code clones (more than 200 SLOC) are less reliable than non-clone modules. We also found that clone-included modules are less maintainable (having greater revision number on average) than non-clone modules; and, modules having larger code clone are less maintainable than modules having smaller code clone.