A thin-film dielectric having a higher dielectric constant than usual ones and not requiring a special single crystal substrate, and a large-capacity thin-film capacitor element using the thin-film dielectric, in which a BaTiO3-based perovskite solid solution and a KNbO3-based perovskite solid solution are alternately formed to form a crystal structure gradient region where a lattice constant continuously changes at the interface, and thus crystal lattice strain occurs, thereby permitting the production of a thin-film dielectric having a high dielectric constant; also, a large-capacity thin-film capacitor element can be produced by using the thin-film dielectric of the present invention.