A rubber composition contains an organopolysiloxane having at least two alkenyl radicals, an organohydrogenpolysiloxane having at least three Si—H radicals, a catalyst and optionally, fumed silica and an acid resistant inorganic filler. On a periphery of one side of a polymer electrolyte fuel-cell separator, the composition forms a seal member which allows for little leaching of non-functional organopolysiloxane fraction and has improved acid resistance and compression set.