An information terminal device includes an acquisition unit that acquires reference information representing a machine-readable feature distributed on a surface of a thing associated with a first user, and collation information representing a machine-readable feature distributed on a surface of a thing associated with a second user, a registration unit that registers the reference information acquired by the acquisition unit from the thing associated with the first user in a storage unit, a collation unit that uses the collation information acquired by the acquisition unit from the thing associated with the second user, and collates the collation information with the reference information, and a judgment unit that judge validity of whether the second user is the first user, based on a result of collation by the collation unit.