A modified butyl rubber composition comprising a butyl rubber modified by mixing, under heating, (a) a compound having a nitroxide-free radical in the molecule thereof, which is stable at an ordinary temperature even in the presence of oxygen, (b) a radical initiator, and (c) a bi- or more-functional radical polymerizable monomer and further by compounded with (d) a bi- or more-functional radical polymerizable monomer, which is different from the ingredient (c).