1. A composition comprising a solvent-soluble organopolysilsesquioxane represented by the formula (I): ##STR2## wherein R and R', which may be the same or different, represent hydrogen, lower alkyl, alkenyl, or aryl groups, wherein at least lower alkyl and alkenyl groups are present in the organopolysilsesquioxane, and n is degree of polymerization, and a compound which generates crosslinking-reaction-active species upon irradiation with light or an ionizing radiation.