A mutual authentication apparatus and method for using the Internet, including a user authentication code generator and a server authentication code generator which execute mutual authentication between a user and a server by generating a server authentication code required for the access to the server using an authentication code generating function based on time information provided from a mobile communication network or a global positioning system (GPS) satellite, accessing the server with the generated server authentication code, generating a user authentication code using an authentication code generating function at the accessed server, and providing the generated user authentication code to the user.