IP server W and gateway server GWS transmit data in HTML format in response to a request from mobile unit MS. At this stage, in the HTML data, the personal identification information request data string is added to a URL of the page corresponding to a service, which requires user authentication. As the HTML data is received by mobile unit MS, the user carries out an inputting operation to designate the URL while the image corresponding to the HTML is displayed on the liquid crystal display unit. Then, mobile unit MS reads out personal identification information from ROM, and transmits to mobile packet communication network MPN. IP server W carries out user authentication on the basis of personal identification information transmitted from mobile unit MS in this manner.