PURPOSE: To hold hierarchically the information of a previous knowledge base, and to recover it automatically, even when the knowledge base to be performed access is changed by providing a stacking means which successively stacks in an index the knowledge base to be performed access by a searching means.

CONSTITUTION: A stacking means which successively stacks in an index 2 knowledge bases(KB1, KB2) to be performed access by the searching means is provided. Then, when the searching means performs access to the object of the knowledge base KB1 stacked at the highest order of the index 2, the stacking means performs access to the object of the other knowledge base KB2. At that time, the stacking means pushes down this knowledge base KB2 on the knowledge base KB1 at the highest order of the index 2, pops up it after the access is completed, and recovers the previous knowledge base KB1 to the highest order in the index 2 in order to resume the access. Thus, the information of the previous knowledge base KB can be held hierarchically and recovered automatically even when the knowledge base KB1 to be performed access is changed.