For the purpose of increasing the mechanical strength of the cured product and fluidity of the composition, these three components may be combined with an organopolysiloxane composition comprising (CH2 ???CH)R2 SiO0.5, R3 SiO0.5 and SiO2 wherein R is a substituted or unsubstituted monovalent hydrocarbon group having no aliphatic unsaturation, the molar ratio of the sum of (CH2 ???CH)R2 SiO0.5 and