PURPOSE: To evaluate influences and effects applied to a plant by an operational manipulation in advance before executing the manipulation.

CONSTITUTION: This system is composed of a knowledge data base device 3 storing data to be the base of plant functional performance evaluation, plant functional performance evaluating device 2 to evaluate the current condition of plant operations based on process data fetched from the plant and a performance evaluation expression read from the knowledge data base device 3 while using functional performance evaluation exponents, manipulation influence evaluating device 4 to evaluate the infleunces and effects, which are exerted to the performance functions of the plant by the manipulation to be executed from now, based on functional performance evaluation exponent data read from the data base device 3 and functional performance evaluation exponent of the current condition calculated by the plant functional performance evaluating device, man-machine interface device 5 to output evaluated results and to input information from operators, and plant functional performance evaluation exponent preparing device 6 to register the functional performance evaluation exponent data of the plant on the knowledge data base device 3 in advance by using a simulator 7.