It is an object of the present invention to provide a rubber composition which has been improved in mechanical properties with maintaining excellent various properties and is favorably used for a fuel cell sealing member, a hard disc drive top cover gasket member and an electric wire connector sealing member. It is another object of the present invention to provide main bodies mounted with the above respective members obtained by the use of the rubber composition. The rubber composition of the invention comprises a specific ethylene/.alpha.-olefin/non-conjugated polyene copolymer in which the non-conjugated polyene is a norbornene-based compound, a specific SiH group-containing compound, and if necessary, an organopolysiloxane. Such a rubber composition of the invention is excellent in mechanical properties and can be particularly favorably used for a fuel cell sealing member, a hard disc drive top cover gasket, an electric wire connector sealing member, etc.