An authentication system according to an embodiment is provided with a service providing apparatus, an IDaaS corporation apparatus, and an authentication providing apparatus. Based on the user ID and SSO request transmitted from the user terminal, the authentication providing apparatus executes authentication processing for the user. If the result of the authentication processing indicates success, the IDaaS corporation apparatus having SSO account information including the SSO account identifier identical to the user ID, permits SSO authentication to be executed for the service identified by the service account identifier included in the service account information associated with the SSO account information. The service providing apparatus transmits information related to the service to the user terminal.