PURPOSE: To avoid the deterioration of the inferring speed by setting the final application time in response to a relevant rule stored in a knowledge base to change said time with application of the rule and subtracting the end time from said changed application time when a series of inferences are through to automatically erase the rule having the difference obtained from said subtraction that is larger than the fixed value.

CONSTITUTION: When a question is given via an input device 4, an inference means 3 informs an application rule to a control means 12 and gives an instruction to a temporary memory means 7 to read the attendant time and then an instruction to a time managing means 8 to output the present time to the means 7. Then an instruction is given to the means 7 to rewrite the time and the means 12 rewrites the time corresponding to the rule by a transfer command. An end signal of the means 3 is sent to the means 12 and the information that has just been read into the means 7 is outputted to an arithmetic means 9. Then a subtraction is carried out by a command and the result of this subtraction is sent to a comparison means 11. The means 11 receives the deleting reference value from a memory means 10 to compare this with said result of subtraction and deletes the rule. In such a way, the undesired rules are deleted out of a knowledge base so that the deterioration of the inferring speed is avoided and the knowledge arrangement is automatically carried out with high efficiency.