To discover high-level knowledge by simply, surely and automatically executing data mining from a database.

This knowledge discovery system 10 for discovering knowledge from data stored in a database 11 to supply the discovered knowledge to a knowledge base 18 provided for an expert system 17 is provided, as a data processing part 12, with: an extraction means 13 for carrying out search from the data stored in the database 11 by using a predetermined predicate keyword to extract the predicate keyword by including sentences before and after it; and a mining means 14 for removing noise by carrying out search refinement and rearrangement of the extracted data. An expert system 17 provided with the knowledge base 18 for storing mining data is provided for the system to discover knowledge based on the data stored in the knowledge base 18.