unit 21 searches the mac conversion table 16 a for sending port with a valid period e g in the event that the received point in time is 01 01 05 jan 1 2006 the start time is 01 01 xx jan 1 2006 and the destination mac address substitute mac address a s 124 and determines whether or not the destination mac address a has been hit s 125 in the event that the destination mac address a has not been hit s 125 no the flow proceeds to processing s 131 in the event that the destination mac address a has been hit s 125 yes the transfer unit 21 rewrites the destination mac address a with the corresponding terminal mac address p in accordance with the mac address conversion table s 126 and the flow proceeds to processing 5133 s 10 the arp substitute response unit 22 performs the same operation as the existing proxy arp process that is to say the arp substitute response unit 22 searches the arp table 23 with ip address 1 1 1 9 which has failed to hit and ends the processing at that time s 11 of the apparatuses which received an arp request frame the server 3 is ip 1 1 1 9 so transmits an arp response frame as the destination.