A transfer device (A; B; C; D) comprising: a transfer management table storage unit to store a transfer management table which indicates transfer destination ports for frames received from another transfer device, the another transfer device being configured to transmit a frame low in importance of received frames without copying this frame and to transmit an important frame of received frames to paths made redundant by copying the important frame, the important frame being a frame higher in importance than the frame low in importance, wherein an important frame is a frame that requires reliability enhancement with reference to information in the frame header, and wherein a transmission side transfer device inserts identifiers including transmission source identification information, path information, and/or order information into the important frame; an adjacent information table storage unit to store an adjacent information table which indicates information about instruments connected to the ports; and a transfer destination management unit to control whether to transfer an important frame received from another transfer device by use of the transfer management table and the adjacent information table; wherein, when the transfer destination management unit receives from another transfer device an important frame having a same data as an important frame already received, the transfer destination management unit obtains information about transfer destination ports from the transfer management table, and further obtains information about instruments connected to transfer destination ports from the adjacent information table, whereby the transfer destination management unit transfers the important frame having a same data as an important frame already received from a port connected to a transfer device, and does not transfer this important frame from a port connected to a subordinate instrument under its own device. a transfer management table storage unit to store a transfer management table which indicates transfer destination ports for frames received from another transfer device, the another transfer device being configured to transmit a frame low in importance of received frames without copying this frame and to transmit an important frame of received frames to paths made redundant by copying the important frame, the important frame being a frame higher in importance than the frame low in importance, wherein an important frame is a frame that requires reliability enhancement with reference to information in the frame header, and wherein a transmission side transfer device inserts identifiers including transmission source identification information, path information, and/or order information into the important frame; an adjacent information table storage unit to store an adjacent information table which indicates information about instruments connected to the ports; and a transfer destination management unit to control whether to transfer an important frame received from another transfer device by use of the transfer management table and the adjacent information table; wherein, when the transfer destination management unit receives from another transfer device an important frame having a same data as an important frame already received, the transfer destination management unit obtains information about transfer destination ports from the transfer management table, and further obtains information about instruments connected to transfer destination ports from the adjacent information table, whereby the transfer destination management unit transfers the important frame having a same data as an important frame already received from a port connected to a transfer device, and does not transfer this important frame from a port connected to a subordinate instrument under its own device.