A management unit (110) which issues a digital certificate to a new
transmission unit (410) includes a wireless communication section for
performing communication in a network (300) and a wire communication
section to which the new transmission unit (410) can be connected; when
the new transmission unit (410) is wire-connected to the management unit
(110), it is judged in accordance with the received device type
information of the new transmission unit (410), whether or not the new
transmission unit (410) is a device having a communication means that can
communicate in the network (300); and if the new transmission unit is
judged as a device having that type of means, the management unit creates
a digital certificate by using a device identifier specific to the new
transmission unit (410), and sends the digital certificate to the new
transmission unit (410).