STAMP (System Theoretic Accident Model and Processes) is one of the theories that has been attracting attention as a new safety analysis method for complex systems. CAST (Causal Analysis using System Theory) is a causal analysis method based on STAMP theory. The authors investigated an information security incident case, 'AIST (National Institute of Advanced Industrial Science and Technology) report on unauthorized access to information systems,' and attempted accident analysis using CAST. We investigated whether CAST could be applied to the cyber security analysis. Since CAST is a safety accident analysis technique, this study was the first to apply CAST to cyber security incidents. Its effectiveness was confirmed from the viewpoint of the following three research questions. Q1:Features of CAST as an accident analysis method Q2:Applicability and impact on security accident analysis Q3:Understanding cyber security incidents with a five-layer model.