In order to reduce load placed on a CPU (central processing unit) in providing SBP-2 (serial bus protocol 2) initiator capability, provided are a sequence control circuit activated by the CPU for controlling a command issue sequence, a packet processing circuit for assembling operation request blocks (ORB) into a transmission packet and extracting a status from a received packet; buffer for storing a command ORB provided by the CPU; a buffer for storing a management ORB provided by the CPU; a buffer for storing a status received for an issued management ORB and providing the status to the CPU; and a buffer for command for storing a status received for an issued command ORB and providing the status to the CPU.