A composition comprising (A) polycycloolefin-functional polysiloxane represented by the following average compositional formula (1) ##STR00001## wherein R.sup.1 is a monovalent organic group which does not have an unsaturated bond and may be different from each other, k is the number of 0 or larger, n is the number of from 0.01 to less than 4, m is the number of from 0 to less than 4, provided that the sum of m and n is larger than 0 and less than 4, (B) a siloxane having an SiH bond represented by the following average compositional formula (2) in such an amount that a molar ratio of the SiH bond to the unsaturated bond in the component (A) ranges from 0.1 to 10, HSiR.sup.2.sub.aO.sub.(4-a)/2 (2) wherein R.sup.2 is a monovalent organic group and a is the number larger than 0 and less than 4, and (C) an addition reaction catalyst in a catalytic amount.