Patent US 11,615,171

A terminal device includes an authentication information acquirer acquiring pieces of authentication information, an authentication determiner determining whether authentication is successful based on, among the pieces of authentication information acquired by the authentication information acquirer, at least one piece, and an authentication information updater, when the authentication determiner determines that authentication is successful, updating pieces of saved authentication information with pieces of authentication information acquired by the authentication information acquirer. The authentication determiner, in the background, determines that authentication is successful when first authentication information acquired by the authentication information acquirer falls below a predetermined authentication threshold and, when the first authentication information is the authentication threshold or more and a predetermined authentication permissible value or less, determines whether authentication is successful based on second authentication information acquired by the authentication information acquirer.