This paper is a comprehensive introduction of the Fault Tree Analysis Method. The F.T.A. has primarily been applied for the purpose of the safety analysis of machine. This paper shows, with an example, the applicability of the F.T.A. to the safety analysis of man machine systems. The F.T.A. model is constructed with events and logical gates. Events are connected by logical gates into a tree structure. In this paper human behavior and the condition which causes human behavior are treated as events.