Data in a storage medium is scanned while a storage device is in an idle state. A proportion of scanned data in entire data in the storage medium is compared with a threshold value. If the proportion of the scanned data is less than the threshold value, an unscanned data in the storage medium is scanned during a time interval between commands received by the storage device from outside. The process is repeated until the proportion of the scanned data is equal to or more than the threshold value.