PURPOSE: To provide the device and method for plant deterioration diagnosis which predict and diagnose the deterioration of an actuator and are suitably improved in the precision of a deterioration diagnosis.

CONSTITUTION: Plant facilities, having the actuator 2 which operates a plant 1, a sensor 3 which detects the operation state of the actuator 2, and a plant controller 4 which generates commands to the actuator 2 by using the information of the sensor 3, is equipped with a response time measuring mechanism 7 which measures the response time of the actuator 2, and a deterioration diagnosing process mechanism 9 consisting of an analyzing mechanism which outputs predicted life by analyzing the response time and an inference mechanism which corrects the predicted life by utilizing a knowledge base 10 and diagnoses the deterioration. Consequently, the precision of the predicted life of the actuator 2 is improved, the precision of the deterioration diagnosis can be improved greatly, and the operation rate of the plant facilities can be prevented from decreasing.