Method for sending notice of failure detection

To reliably detect a failure even when the failure occurs at a part of a line or in a mating apparatus, a transmission control portion transmits a link-up signal for confirming connection by the mating apparatus to a transfer path during the period in which data is not transmitted, in accordance with the instruction from a line control portion. Receiving the link-up signal flowing through the transfer path, the reception control portion confirms that no failure exists in the mating apparatus and in the transfer path or in the apparatus, and sends this notice to the line control portion. When not receiving the link-up signal, the line control portion reports the failure to the mating apparatus by intentionally stopping transmission of the link-up signal through another transfer path. To examine the operation condition of the line, a specific signal that can be sent during the period in which the data is not transmitted, and represents whether or not the line is utilizable is used as the link-up signal.