Provided is a plant state monitoring method which monitors an operation state of a plant by using the Mahalanobis distance based on the plant state amount. The method creates a first unit space as a set of data used to be a reference when judging whether the plant operation state during a start operation period is normal according to the state amount in the plant start operation period. The method also creates a second unit space as a set of data used to be a reference when judging whether the plant operation state during a load operation period is normal according to the state amount in the plant load operation period.