When verifying rapidly the equivalence between source codes with respect to refactoring, the present invention performs two types of verification: verification based on structural comparison using structure graphs obtained by analyzing the source codes, and verification based on symbolic execution. If the structural comparison using the structure graphs can verify that the structures are identical with each other, then symbolic execution is not performed. Further, before the verification based on the structural comparison, the structure graphs of the source codes before and after refactoring are normalized on the basis of normalization information, which is defined for each refactoring pattern, and thereby adjusted so that the resulting structures are identical with each other when the refactoring is valid. Further, the structure graphs before and after the refactoring are subjected to abstraction before being subjected to symbolic execution for verification, thereby limiting locations to which symbolic execution is to be applied.