PURPOSE: To lessen an operator's surveillance and operational work load by inferring a pattern matching and the like, and by deciding a plant condition using a device for deciding plant conditions.

CONSTITUTION: A processing amount of the plant and plant informations such as conditions of equipments' start-up, shut-down and the like are collected with a certain time duration on an online basis. Those plant informations are inferred using a knowledge data stored in a knowledge data base 2 for decision of a plant condition at a device 3 for deciding plant conditions, and after that, the plant condition informations are summed up and unified. Then, based on results of decision obtained by the device 3 for deciding plant conditions, on-line data of the plant from a process input/output device 1, are arranged and compiled at an information generator 4 for a display drive. Based on the arranged and compiled form, an image and surveillance parameters are output on the display unit 5 and are shown to operators.