A curable composition may include 100 parts by weight of a polyisobutylene-based polymer (A) having 1.2 or more (meth)acryloyl groups per molecule, 15 to 900 parts by weight of a polyisobutylene-based polymer (B) having 0.5 to 1.0 (meth)acryloyl group per molecule, and 0.001 to 50 parts by weight, per 100 parts by weight of the total weight of the polymer (A) and the polymer (B), of a polymerization initiator (C). Each of the polymer (A) and the polymer (B) may have a number average molecular weight of 500 to 500,000 as measured by size exclusion chromatography based on polystyrene standards, and a molecular weight distribution (weight-average molecular weight Mw)/(number-average molecular weight Mn) of 1.0 to 2.0.