PURPOSE: To obtain a diagnosis type expert system regardless of the production rule, etc., by using a decision table which produces a knowledge base.

CONSTITUTION: A decision table display processing part 11 displays a decision table 17 on the screen of a display 16 at production/editing of a knowledge base 15. An input device 18 supplies the item information on the knowledge into each input field based on the displayed table 17. Thus a decision table item input processing part 12 stores the input result of the item information into a table data memory part 14. A table/knowledge base conversion processing part 13 produces the base 15 to be handled by an inference engine in an expert system based on the contents of the table 17 stored in the part 14.