In a disk device, when processing is resumed after replacing the disk or cutting off the power supply, in order to acquire disk-related information in a short period of time and reduce the start-up time, the effectiveness of management information recorded on the disk is judged, the finding of the judgment is stored into a memory means capable of storing even when power supply is off, the management information is read out of some of the recording areas of the disk in advance of a recording or reproducing operation if the stored information on the finding of the judgment indicates effectiveness for the inserted disk.