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).