A computer system 1 has a PBX 2, a terminal device 3, and a management server 4. The PBX 2 has a line controller, an extension controller 2b, an interface, a memory that stores identification information and an enciphered file, a decoder that decodes the enciphered file, a comparator that compares the identification information and the enciphered file decoded by the decoder, and a controller. The terminal device 3 reads the identification information from the PBX 2 and transmits that to the management server 4. The management server 4 has an authenticator that receives a request for authentication when the terminal device 3 logs in, an issuer that issues the enciphered file according to the identification information, and a database that stores the issue date in correlating with the identification information.