In a router provided on a boundary between the Internet and an intranet and used for receiving an incoming first packet and then passing on the first packet to a destination router indicated by a destination address of the first packet, the router includes a decapsulation unit for carrying out a process of decapsulating the first packet into a second packet in the case of a predetermined address specified in the first packet as the destination address, and a first judgment unit for forming a judgment as to whether or not a user transmitting the first packet is an authorized user.