An apparatus comprising (a) a steam-induction-heating apparatus, which comprises a first container having an inlet and an outlet, a high-frequency induction coil wound around said first container, and a member or members placed in said first container with steam permitted to pass therethrough for being induction-heated by said high-frequency induction coil; and (b) an electric discharge treatment apparatus located downstream of said induction-heating apparatus, which comprises a second container having an inlet and an outlet, and at least a pair of electrodes disposed in said second container for subjecting induction-heated steam to an electric discharge treatment; superheated steam exiting from the outlet of said induction-heating apparatus being converted to the activated steam by an electric discharge treatment in said electric discharge treatment apparatus.