A positive-tone radiation-sensitive composition including: (A) at least one compound selected from the group consisting of a hydrolyzable silane compound represented by general formula (R¹)_pSi(X)_4-p (wherein R¹ is a non-hydrolyzable organic group having 1 to 12 carbon atoms, X is a hydrolyzable group, and p is an integer from 0 to 3), hydrolyzates thereof and condensates thereof; (B) a photoacid generator; and (C) a basic compound. A cured product that is excellent in terms of pattern precision and so on can be obtained by using the composition. The composition can be used as a material for forming optical waveguides.