The present disclosure provides a method for superseding a log-in through PKI-based authentication with respect to a log-in request of a user by using a blockchain database. According to the method, once authentication request information requesting superseding of a log-in through an authentication app is obtained from a service provision app executed on a user terminal, a service provision server transfers authentication request response information to the service provision app and, after an authentication redirection request thereof is transferred to the authentication app and then server challenge request information is obtained, server challenge request response information is transferred to the authentication app, an authentication result message including information on whether certificates of the server and the app are valid is obtained from an authentication server, a predetermined access token is transferred to the service provision app, and thereby the log-in is handled such that a service can be used.