PURPOSE: To automatically prepare index to be frequently used by recognizing the access state of index to be used by a user.

CONSTITUTION: The frequency to be used of the index which is registered in an index registration part 21 which is used for retrieving terminal equipments A and B is calculated. A changeover discrimination part 24 selects the index to be more frequently used than the frequency to be used which is set in advance from among the calculated frequency to be used of each index. A changeover part 26 updates and registers the selected index instead of the index registered in the index registration part 21.