Disclosed is an electrolyte membrane which exhibits high ion conductivity even under high temperature no humidification conditions. Specifically disclosed is an electrolyte membrane comprising a complex oxoacid solid, which contains two or more kinds of oxoacid groups, hydrogen and at least one element selected from Mg, Ca, Sr and Ba, and a reinforcing material contained in the solid for improving the mechanical characteristics of the solid. The reinforcing material is composed of a polymer material or an inorganic material.