Abstract: In response to an alarm, an inference engine in a plant operation support system calculates reliabilities of the appearance of phenomenons based on process data from the plant and a knowledge data base. The knowledge data base refers to diagnosis information and a fault tree. The value obtained from the calculation is compared with a threshold value inherent to each phenomenon. If the calculated value is larger than the threshold value, it is determined that the phenomenon has appeared. The appeared phenomenons are successively displayed on the screen of a display unit in order starting from the alarm phenomenon down to the subordinate phenomenons. An operation guidance corresponding to the displayed phenomenon is retrieved from a guidance data base and is displayed on the display unit.