To assure that a system is safe, several tasks must be performed, resulting in the compilation of several safety-related documents. It is important that all the information related to safety is developed and maintained consistently. Because of retirements and changes to different department, it is difficult for engineers to pass on their knowledge. To ensure consistency throughout a project lifecycle notwithstanding program-level changes, a method to express the relationship between the safety information described in different documents is needed. Moreover, to address not only specifications but also bases for design formulated by a designer, design knowledge is accumulated together. We aim to improve the understanding of the safety design. In this study, we add two rules to the D-Case in order to achieve three goals. To verify the effectiveness of our method, these rules were applied to a rocket system.