Change bits are provided in correspondence with storage information units (blocks or pages) of a storage, and indicate whether or not a "store" operation has been performed into the corresponding storage information units. The change bits are retained in a first retention device in correspondence with the storage information units, and a copy of the change bits in the first retention device is retained in a second retention device. When the store operation is performed to effect a storing of data into the storage, a control device refers to the change bit retained by the second retention device and controls updating of the change bit of the first retention device in accordance with the indication of the change bit referred to.