Abstract: A curable composition comprising the components (A), (B), (C) and (D), viz.(A) an isobutylene polymer which contains in a molecule, at least one alkenyl group capable of hydrosilylation reaction;(B) a curing agent which contains at least two hydrosilyl groups in a molecule;(C) a hydrosilylation catalyst; and(D) an organic compound which contains in a molecule, at least one alkenyl or alkynyl group capable of hydrosilylation reaction.This composition is excellent in workability because of its low viscosity, and has such advantages that the cured product thereof has large strength and further that the weight loss by heating is small. Since a large amount of an inorganic filler can be blended with this composition, it is possible to obtain a composition having high thixotropy and little sagging.