This study proposes an assurance case description method to reduce misunderstanding caused by the difference of grasping the objects managed in various departments as a monolithic system or a System-of-Systems (SoS). The assurance case method describes various misunderstandings and the classification of patterns of how misunderstanding occurs. We derived steps from the classification of patterns to eliminate the differences in grasping the managed objects. Six researchers in systems engineering confirmed the validity and necessity of the proposed approach for grasping the objects managed in various departments as a monolithic system or a SoS, and the five steps needed to reduce the four patterns of misunderstanding. In addition, this study verified through interviewing experts Understanding, Utility, and Effectiveness of the proposed assurance case description method. This study concludes with future research topics.