A relay unit capable of inquiring of a DNS comprises: an inquiry section which transfers an acquisition request of a transmission destination terminal address, sent from a transmission source terminal, to the DNS, and acquires the transmission destination terminal address and a transmission destination label corresponding to the transmission destination terminal address and transmission destination terminal; a label creation section which creates a unique relay label; a label storage section which correlates the created relay label to the acquired transmission destination terminal address and transmission destination label, and stores thus obtained label; a label transmission section which transmits the relay label and a private address of the relay unit to the transmission source terminal; and a determination section which, if a packet containing the relay label is received from the transmission source terminal, determines that the transmission destination terminal address and transmission destination label corresponding to the relay label are an address and a label related to a transmission destination or a relay destination of the packet.