Provided is an electrolyte membrane exhibiting high proton conductivity even in low humidity. The electrolyte membrane comprises a composite membrane provided with a polyolefin microporous membrane that has an average pore size of 1 to 1000 nm and a porosity of 50 to 90% and that can be impregnated with a solvent having a surface free energy of 28 mJ/m2 or higher, and an electrolyte containing EW250-850 perfluorosulfonic acid polymer packed into the pores of the polyolefin microporous membrane, the thickness of the composite membrane being 1 to 20 μm.