Provided is a method for automatic user authentication, including: (a) a first step in which a user credential is stored on a website in the course of membership registration, wherein the user credential includes a given mobile phone number; (b) a second step in which a service application is downloaded onto a first mobile terminal, wherein the first mobile terminal is linked to the website server; and (c) a third step in which the service application is activated on the first mobile terminal through automatic user authentication without an additional membership registration process.