Creating a plaintext index from a text that is extracted from a file presents the risk of a leak of confidential information from the created index. To address this problem, provided is a computer system which has a computer, a storage subsystem coupled to the computer, and a network coupling the computer and the storage subsystem. The computer has an interface coupled to the network, a first processor coupled to the interface, and a memory coupled to the first processor. The storage subsystem has a disk device which stores data. A storage area of the disk device is divided into a plurality of storage areas including, at least, a first storage area and a second storage area. The first processor reads a part of data stored in the first storage area, encrypts the part of data read from the first storage area when the data stored in the first storage area is judged as encrypted data, and writes the encrypted part of data in the second storage area.