A disk array apparatus is provided, which has dual FC-Als to each of which an enclosure service unit connects, and is capable of performing loop diagnoses including detachment and reconnection of devices even when abnormalities occur in all loops. The enclosure service units can communicate each other through an interface. Heartbeat monitors, when detecting that heartbeat commands transmitted through the FC-Als have ceased in both loops, detach all disks connecting to the FC-Als to thereby resolve the loop abnormalities. Then, loop diagnostic units cooperatively perform loop diagnoses including reconnection of disks to the FC-Als, identify faulty disks, detach the faulty disks, so as to continue the operation of the system.