A curable composition which comprises at least one of the following (A), (B), and (C) and further contains (D) (provided that when (C) is not contained, both (A) and (B) are in the composition. (A): A silicon-containing polymer in which the content of components having a weight-average molecular weight of 1,000 or lower is 20 wt. % or lower and which has a reactive group A′ and one or more Si—O—Si bonds. (B): A silicon-containing polymer in which the content of components having a weight-average molecular weight of 1,000 or lower is 20 wt. % or lower and which has an Si—H group and one or more Si—O—Si bonds. (C): A silicon-containing polymer in which the content of components having a weight-average molecular weight of 1,000 or lower is 20 wt. % or lower and which has a reactive group A′, an Si—H group, and one or more Si—O—Si bonds. (D): A catalyst for curing reaction which is a platinum catalyst. The reactive group A′ is any of Si—R1, Si—O—R2, and Si—R3—OCOC(R4)═CH2, provided that R1 and R2 each is alkenyl, R3 is alkylene and/or arylene, and R4 is hydrogen or methyl.