To obtain a composition, which is able to provide a cured product well bonded to a hard-to-bond resin, by comprising specific types of organopolysiloxane, organohydrogenpolysiloxane and (meth)acryloxyalkyl- modified organopolysiloxane, and platinum in specified amounts, respectively.

This composition comprises 100 parts by weight of an organopolysiloxane represented by the average compositional formula: R1aR2bSiO(4-a-b)/2 and having two or more alkenyl groups bonded to the Si atom in the molecule, an organohydrogenpolysiloxane represented by the average compositional formula: R3cHdSiO(4-c-d)/2 and having two or more H atoms bonded to the Si atoms in an amount sufficient to provide 0.4 to 10 H atoms bonded to the Si atoms per unit alkenyl group in the composition, 0.0001 to 3 parts by weight of a (meth)acryloxyalkyl-modified organopolysiloxane having a viscosity of 5 to 10,000 cps (25°C), and platinum (compound). In the formulas, R1 and R3 independently represent an aliphatic unsaturation bond-free monovalent hydrocarbon group; and R2 represents alkenyl.