To solve the problem that processing becomes complicated because data have to be transmitted after performing broadcast transmission and newly resetting an address in all of cases irrespective of whether an IP (Internet protocol) address is newly registered or has been already registered or whether the IP address has been changed.

MAC address inquiry procedures 5-1 accesses a device to request a MAC (media access control) address return on the basis of the IP address of the device in response to a line connection request. IP address/MAC address storing procedures 5-2 associates the IP address with a MAC address and stores them as a device address when the MAC address return is received. Thus, even when the IP address has been changed, data can be transmitted to the changed IP address.