A disk recording/reproduction device ideal for processing audio-visual data streams requiring real-time characteristics. A disk device has an operating mode ideal for recording and reproduction of motion image and audio data requiring real-time continuity by limiting the time needed for error processing, and the host incorporating the disk device can easily monitor the real-time continuity. The host can specify the urgency of individual recording and reproduction instructions to the disk so that control of real-time continuity can be actively implemented in better detail. A means for assigning a total number to a plurality of simultaneously recorded or reproduced data streams as well assigning individual data stream numbers allows informing the disk device of the throughput required by the host.