We deal here with diagnosis for failures of series/parallel structure system. The conventionals have assumed that the system failure is caused by a single minimal cut set (MCS). The purposes of this paper are to propose a new diagnosis method to identify all MCSs by utilizing the series/parallel structure and repair information without requiring an excessive number of inspections. Moreover, by applying our method to several types of series/parallel structure system, and to system having some kinds of probability distributions, desirable system for our method are persuaded. We evaluate not just the number of inspections but also the cost of diagnosis, and show the condition under which our method is effective.