To provide a rubber composition which shows a higher heat resistance compared to that of a peroxide-crosslinked butyl rubber and exerts a long life even at elevated temperatures, an electrolytic capacitor sealing cap and its manufacturing process.

A hydrogenated silane-crosslinkable rubber polymer is crosslinked with a hydrogenated silane, and divinyl benzene is re-crosslinked with a peroxide. A rubber composition, prepared via a preparation process comprising steps wherein the hydrogenated silane-crosslinkable rubber polymer is crosslinked with a hydrogenated silane and the crosslinked product is supplemented with divinyl benzene and then re-crosslinked with a peroxide, and an electrolytic capacitor sealing cap made of this have improved physical and life properties compared to those of conventional compositions and sealing caps.