By incorporating (A) a saturated hydrocarbon polymer having more than one alkenyl group capable of being hydrosilylated in each molecule and having a molecular weight of 2,000 to 50,000, (B) a component comprising a compound containing more than two silicon atom-bound hydrogen atoms, on average, in each molecule, (C) a compound having, within the molecule, one alkenyl group capable of being hydrosilylated, (D) a compound having, within the molecule, at least two alkenyl groups capable of being hydrosilylated and having a molecular weight less than 2,000, and (E) a hydrosilylation catalyst, a composition having good mechanical characteristics, a high gel fraction and a small heating loss can be obtained. As a result, the invention can provide a composition to secure the uniformity within a wide range of selection not depending on its viscosity, because it is markedly relaxed the restriction on SiH-containing compounds that are curing agents possibly becoming a cause of said insufficient curing.