Summary: Assurance improvement is the most challenging task both in railway and space areas, even though their required technologies are different. It can be said that the railways have been achieving the extremely high assurance level through their long history and experience, while the space area has not accumulated enough experience in addition to their requirement of highly advanced technology. As a result, the latter places more emphasis on theoretical analyses for ensuring the assurance. This paper introduces the two different approaches of railway and space areas toward assurance, and compares the two methods.