According to one embodiment, there is provided an Internet Protocol communication system provided with terminal devices configured to mutually communicate with one another via an IP network and a server unit which performs digest authentication in response to authentication requests transmitted from the terminal device. The server unit comprises an authentication processing module which transmits challenge values to terminal devices of authentication request sources, and verifies response values returned to the challenge values, and a determination module which determines results of the digest authentication on the basis of the results of the verification. At least one of the terminal devices comprises an authentication client module which generates the response values by using a defined algorithm in accordance with user passwords input by users, and with device passwords stored in advance, and returns the response values to the server unit.