The relay device, comprising an address correspondence holding section which holds, for each of the terminals, the IP address and real MAC address of the terminal and a virtual MAC address being a MAC address that is virtual; a proxy reply section which, when a first ARP request frame that seeks the acquisition of the virtual MAC address from the terminal is received, reads the virtual MAC address of the corresponding terminal from the address correspondence holding section and sends back the virtual MAC address to the terminal; a MAC address conversion section which receives a first frame addressed to the virtual MAC address when the first frame is sent and received between the terminals, performs conversion of the virtual MAC address and real MAC address for the MAC address of the first frame, and sends back a converted second frame; and a security check section.