An electrolytic capacitor sealer capable of fulfilling the requirement such as long-term heat resistance without risk of reversion due to crosslinking with an organic peroxide and an electrolytic capacitor using such a sealer are provided. A sealer is formed of a modified butyl rubber composition obtained by using an organic peroxide to crosslink a modified butyl rubber composition which has beenmodified by reacting butyl rubber with a compound (a) having in the molecule a nitroxide free radical that is stable at room temperature even in the presence of oxygen, a radical initiator (b), and abifunctional or higher radical polymerizable monomer (c), and of a modified butyl rubber composition obtained by compounding a radical polymerizable monomer (c) with a modified butyl rubber composition that has been obtained by reacting butyl rubber with a compound (a) having in the molecule a nitroxide free radical that is stable at room temperature even in the presence of oxygen and a radical initiator (b), and then crosslinking the composition thus obtained with an organic peroxide.