An electrolyte membrane includes inorganic oxide particles having proton conductivity, and an organic resin or an inorganic matrix component. The inorganic oxide particles having proton conductivity preferably comprise hydrated antimony oxide particles represented by the following formula (1) and have an average particle diameter of 5 to 50 nm, and the content of the hydrated antimony oxide particles is preferably in the range of 5 to 80% by weight in terms of an oxide (Sb.sub.2O.sub.5); Sb.sub.2O.sub.5.nH.sub.2O (1) wherein n is 0.1 to 5. The electrolyte membrane provides a fuel cell which exhibits high cell performance even after prolonged operation and/or at high operating temperatures.