In this study, a relationship between nuclear safety and inspection/monitoring was discussed. As a result of consideration, the followings were made clear. (1) Nuclear safety is secured by the combination of safety functions of nuclear plant system and plant managements by the human system including plant operator’s organization. (2) Nuclear plant systems should be managed for each safety system because safety functions are fulfilled for each safety system. (3) The integrity of concerned system in the future can be evaluated only by the combination of inspection/monitoring and ageing degradation evaluation. (4) Inspection and monitoring is positioned as one of activities in plant management by the human system. (5) The two step method to evaluate the system integrity by using the large and local area evaluation approach can be considered to be reliable and reasonable. (6) This study specified a method to ensure reliability of a system in nuclear power plant by making a comparison between maximum acceptable aging defect of each component and capability of inspection technology applied to the component.