A composite electrolyte membrane for decreasing the crossover of polar organic fuel and a fuel cell employing the membrane are provided. The composite electrolyte membrane includes a modified silica in which silicon atoms have substituents as represented by formula 1 and formula 2; and an cation exchange group-containing polymer:Formula 1 --R.sub.1--SO.sub.3XFormula 2 --R.sub.2--S--S--R.sub.3-- wherein, R.sub.1 is an alkylene group with 2 7 carbon atoms, X is a hydrogen atom or an alkali metal, R.sub.2 and R.sub.3 are each independently an alkylene group with 2 7 carbon atoms.