Abstract: An abnormality-monitoring method and a computer program for plant abnormality monitoring with which it is possible to reliably detect a change in the condition of a plant even when the condition changes due to maintenance, etc., and to appropriately update reference data as needed and constantly monitor abnormalities with high accuracy on the basis of the detected change in the condition. To achieve this purpose, the Mahalanobis distance of an aggregate of collected measurement data and reference data is calculated, the respective dispersion values of the reference data in a reference period and the measurement data in the most recent fixed period are calculated and compared per fixed interval for the calculated Mahalanobis distance, and the reference data is updated when the result of the comparison exceeds a criterion serving as a threshold value.