PLANT OPERATION ASSISTANCE DEVICE AND OPERATION ASSISTANCE METHOD

A plant is stopped at an early stage when an abnormal event occurs to avoid an increase in damage cost due to the abnormality propagation. An operation assistance device 200 includes a diagnosis unit 500 which diagnoses an operation state of a plant 100 based on operation data 3, an abnormality propagation prediction unit 600 which predicts equipment to which an abnormality spreads and time on the basis of a physical model based on a diagnosis result obtained by the diagnosis unit 500, a cost calculation unit 700 which predicts a temporal change of a damage cost on the basis of a result of the abnormality propagation prediction unit 600, and a plant stop determination unit 800 which generates a signal to stop the plant 100 in a case where the damage cost is equal to or greater than a damage cost due to a predetermined emergency stop of the plant 100.