A most recent value of data fetched from a plant facility is temporarily stored and extracted and then stored in a plant history data table. On another front, a monitoring point for monitoring a plant error is registered in response to an operator request. A monitoring point is used as a key to select and register a main variable point having a strong correlation from the plant history data table. A statistical upper/lower limit function is defined on the basis of time-series plant data of monitoring points and main variable points. By referring to information in a table in which a limit value function is stored, the limit value is periodically matched with the content of a plant-data most-recent-value table. If the most recent value is outside the limit range, deviation information is stored in a determination result table. If a new plant error occurs, an error signal and the content of the determination result table is outputted and displayed.