PURPOSE: To shorten the inference time by reducing the modification of the data of a data base after storing the data in a knowledge base or the accessing amount to the data base.

CONSTITUTION: A time flag for defining accessing intervals and preceding access time indicating the accessed time are provided as the fact data of a knowledge base. In addition, a means which compares the difference between the preceding accessing time and the current accessing time with the value of the time flag is provided. Then control is made so that the value of the time flag can be utilized and, in addition, the data in a fact memory can be utilized as they are in accordance with the compared result or the data newly fetched by accessing the data base.