The present invention relates to a method and system that can safely and conveniently perform user authentication by a service provider server and at a public terminal using one-time identification information. According to the present invention, when user authentication is performed using a public terminal to receive an Internet service provided by a service provider, personal identity information to be provided to the service provider can be prevented from being leaked due to fishing or hacking while the personal identity information is input. Therefore, the user can be safely and conveniently authenticated by the service provider.