PURPOSE: To easily hierarchize classification by adding new classification on an existing classification title as high-order concept for the classification.

CONSTITUTION: When an instruction to convert the structure of a data base is issued first in step 151, data of middle classification as a high-order hierarchy in existing classification is added in steps 152 to 154. An input image of middle classification title is displayed in step 155, and an inputted classification title in step 156 is set in step 157. Then, data of large classification of further higher- order hierarchy is added in steps 158 to 160, and when the classification title is inputted to the input image of large classification title displayed in step 161 in step 162, the large classification title is set in step 163, and after that, tree structure is displayed in step 165.