A sealing material comprising (A) an organopoly-siloxane containing at least two alkenyl groups in a molecule, (B) a resinous copolymer composed mainly of R.sub.3SiO.sub.1/2 and SiO.sub.2 units in a molar ratio of 0.5-1.5:1, wherein R is a monovalent hydrocarbon group, and containing 5.times.10.sup.-3 to 1.times.10.sup.-4 mol/g of vinyl groups, (C) an organohydrogenpolysiloxane containing at least two Si--H groups in a molecule, (D) fumed silica having a specific surface area of 150-400 m.sup.2/g, and (E) an addition reaction catalyst is suited for sealing one side edge of a polymer electrolyte fuel-cell separator since it has excellent acid resistance and improved compression set in acidic solutions, and exerts satisfactory seal effects.