An abnormality diagnosis system for diagnosing an abnormality event of a nuclear power plant on the basis of abnormality indication detection results in an abnormality indication monitoring system configured to detect an abnormality indication of the nuclear power plant by using a correlation value indicating a correlation between a plurality of measurement parameters measured in the nuclear power plant, the abnormality diagnosis system comprising: an abnormality diagnosis control unit configured to acquire a parameter transition which is a transition in a measurement parameter in which the correlation value is an abnormal value in a monitoring period from a non-detection timing to an indication detection timing, and a parameter contribution of the measurement parameter contributing to a change in a detection value for detecting an abnormality indication in the monitoring period, as the abnormality indication detection results, the indication detection timing being a time at which an abnormality indication is detected, and the non-detection timing being a time earlier than the indication detection timing; and a database configured to store an abnormality event of the nuclear power plant, an abnormality indication facility correlated with the abnormality event, the parameter transition for determination which is generated on the basis of the past operation history of the nuclear power plant and is correlated with the abnormality event, and the parameter contribution for determination which is generated on the basis of the past operation history of the nuclear power plant and is correlated with the abnormality event, wherein the abnormality diagnosis control unit is configured to determine whether or not the parameter transition and the parameter contribution included in the abnormality indication detection results acquired from the abnormality indication monitoring system respectively match the parameter transition and the parameter contribution stored in the database, and to specify an abnormality event and an abnormality indication facility correlated with the parameter transition and the parameter contribution determined as matching, characterized in that the abnormality indication monitoring system is configured to derive the correlation values respectively indicating correlation strengths of two measurement parameters among the plurality of the measurement parameters, to use the correlation value obtained by adding all the derived correlation values of the measurement parameters together, as a monitoring indicative value, and to determine that there is an abnormality indication in a case where the monitoring indicative value exceeds a threshold value set in advance. an abnormality diagnosis control unit configured to acquire a parameter transition which is a transition in a measurement parameter in which the correlation value is an abnormal value in a monitoring period from a non-detection timing to an indication detection timing, and a parameter contribution of the measurement parameter contributing to a change in a detection value for detecting an abnormality indication in the monitoring period, as the abnormality indication detection results, the indication detection timing being a time at which an abnormality indication is detected, and the non-detection timing being a time earlier than the indication detection timing; and a database configured to store an abnormality event of the nuclear power plant, an abnormality indication facility correlated with the abnormality event, the parameter transition for determination which is generated on the basis of the past operation history of the nuclear power plant and is correlated with the abnormality event, and the parameter contribution for determination which is generated on the basis of the past operation history of the nuclear power plant and is correlated with the abnormality event, wherein the abnormality diagnosis control unit is configured to determine whether or not the parameter transition and the parameter contribution included in the abnormality indication detection results acquired from the abnormality indication monitoring system respectively match the parameter transition and the parameter contribution stored in the database, and to specify an abnormality event and an abnormality indication facility correlated with the parameter transition and the parameter contribution determined as matching, characterized in that the abnormality indication monitoring system is configured to derive the correlation values respectively indicating correlation strengths of two measurement parameters among the plurality of the measurement parameters, to use the correlation value obtained by adding all the derived correlation values of the measurement parameters together, as a monitoring indicative value, and to determine that there is an abnormality indication in a case where the monitoring indicative value exceeds a threshold value set in advance.