A composition comprising a fluid, silica precursor (A) obtained by subjecting alkoxysilane(s) to hydrolysis/polycondensation in the presence of an acid catalyst, a basic compound (B) having a basic hydroxyl group and/or a basic nitrogen atom, wherein the pH of a 0.1 N aqueous solution of compound (B) is 11 or more, and the vapor pressure of compound (B) is 1.3 kPa or lower at 100.degree. C., and an organic compound (C) having a boiling point of 100.degree. C. or higher, compound (C) being compatible with silica precursor (A), wherein the amount of compound (B) is from 0.0015 to 0.5 mol, in terms of the total molar amount of the basic hydroxyl groups and the basic nitrogen atoms in compound (B), per mole of Si atoms contained in silica precursor (A).