PURPOSE: To speed up an inference processing by storing the largest degree of conviction of a rule which is successful in matching in a conviction degree storage area.

CONSTITUTION: An inference processing part 3 matches a condition sentence according to fact data in a fact data storage area(SA) 2c and the rule in a knowledge base 4. The degree of conviction of the matched condition part is compared with the degree of conviction stored in a conviction degree SA 2b and when the degree of conviction of the newly matched condition part is larger or equal, the stored degree of conviction is updated into the degree of conviction of the matched condition part. Further, the rule pointer of the rule is stored in a rule pointer SA 2a. Then the product of the degree of conviction of each condition sentence included in the condition part having the maximum degree of conviction and the degree of conviction of the conclusion part of the rule containing the condition part of the maximum degree of conviction becomes the weight of each conclusion sentence included in the rule and the weighted conclusion sentences are stored in a conclusion SA 2d.