A failure of components or a disturbance of systems can cause a lot of nuisance alarms because of physical or logical connections between components or systems. And so many nuisance alarms can lead to a serious human burden for an operator like the TMI accident. After the TMI accident, many alarm reduction systems and diagnostic systems have been studied to reduce nuisance alarms and to detect the causes of an abnormal state. These systems provide an operator with information on significant alarms or causes of an abnormal state for an operator to identify that state. So, most of these studies have focused on high level information like physical or logical states of a failure rather than the state of process signals as detailed causes of an alarm. In this paper, an operator-aid system is proposed for tracking the logics of an alarm, finding the causes of an alarm, displaying the highlighted alarm procedure related to the causes, and suppressing and filtering nuisance alarms due to the physical or logical connections between components or systems in an abnormal state. The system can be used by an operator to identify the detailed causes of an alarm without checking all the causes of the candidates by alarms. The proposed system will be applied to a nuclear power plant of a CANDU type, Wolsong 3&4 Nuclear Power Plant.