In the key management software having a key database with a tree structure, a high-speed data encryption/decryption process is achieved by changing the tree structure without reducing the security strength when deleting or adding a key from/to the tree structure. The key management software 135 having the key database 146 with the tree structure, when deleting or adding a key from/to the tree structure, refers to the encryption strength comparison table 170 and the process time comparison table 180 to change the tree structure without reducing the security strength. This reduces the number of times an encrypted key is loaded onto the encryption/decryption processing device 134 during the data encryption/decryption process, thus achieving a high-speed data encryption/decryption.