Provided is an electrolyte membrane that exhibits a high ion conductivity even under high-temperature and non-humidified conditions. This electrolyte membrane includes: a composite oxoacid solid including at least two kinds of oxoacid groups, hydrogen, and at least one element selected from the group consisting of Mg, Ca, Sr and Ba; and a reinforcing material that is included in the solid and improves the mechanical property of the solid. The reinforcing material is made of a polymer material or an inorganic material.