A radiation-curable composition containing (A) hydrolyzates of hydrolyzable silane compounds represented by general formula (1): (R.sup.1).sub.p(R.sup.2).sub.qSi(X).sub.4-p-q (wherein R.sup.1 is a non-hydrolyzable organic group having 1 to 12 carbon atoms that contains fluorine atoms, R.sup.2 is a non-hydrolyzable organic group having 1 to 12 carbon atoms (but excluding ones that contain fluorine atoms), X is a hydrolyzable group, p is an integer of 1 or 2, and q is an integer of 0 or 1) and condensates of such hydrolyzates, and (B) a photo acid generator, wherein the ratio of silanol (Si--OH) groups in the composition is 0.1 to 0.5 out of all the bonding groups on Si. With such a composition, the waveguide loss is low for light having a wavelength in a broad range from the visible region to the near infrared region, and moreover the cracking resistance, the patterning ability upon irradiation with radiation, and so on are excellent.