A rubber composition includes (a) a cross linkable fluoro
elastomer, (b) a reactive
fluorine-containing compound (excluding the cross linkable fluoro
elastomer (a)) having a
divalent perfluoropolyether structure or
divalent perfluoroalkylene structure and containing, in the terminal or a
side chain thereof, two or more of alkenyl groups capable of conducting an
addition reaction at least with a hydroxyl group in the following
organosilicon compound (c), and (c) a reactive
organosilicon compound having two or more of hydroxyl groups in the molecule thereof and being capable of conducting an
addition reaction with an alkenyl group in the above compound (b), in such amounts wherein the total amount of (b) and (c) is 1 to 10 parts by weight relative to 100 parts by weight of said rubber (a), and further includes a vulcanizing agent and optionally a crosslinking agent; and a sealing agent for a
plasma treatment device, which is produced by subjecting said composition to a primary
vulcanization and forming, and then subjecting unreached components in the
resultant formed article to a secondary
vulcanization in a vacuum oven, to thereby reduce the amount of a gas which is an unreacted component and may be discharged.