To prepare a sealing material layer by using a photosetting composition, which is usable under a high-temperature condition in a cured state without losing the function as an elastic sealing material.

The subject radiation curable composition is composed mainly of (A) 100 pts.wt. of a polyisobutylene compound expressed by formula (a) (R1 and R2 are each an alkyl or an alkenyl; and (m) is 8-800) and having a molecular weight of 500-50,000, (B) 20-60 pts.wt. of a hydrogenated polybutadiene compound having (meth)acrylate group and (C) a photo-radical polymerization initiator.