It is an object of the present invention to provide a proton-conducting membrane, excellent in resistance to heat, durability, dimensional stability and fuel barrier characteristics, and showing excellent proton conductivity at high temperature. It is another object of the present invention to provide a method for producing the same. It is still another object of the present invention to provide a fuel cell using the same. The present invention provides a proton-conducting membrane, comprising a carbon-containing compound and inorganic acid, characterized by a phase-separated structure containing a carbon-containing phase containing at least 80% by volume of the carbon-containing compound and inorganic phase containing at least 80% by volume of the inorganic acid, the inorganic phase forming the continuous ion-conducting paths. The present invention also provides a method for producing the above proton-conducting membrane, comprising steps of preparing a mixture of a carbon-containingcompound (D) having one or more hydrolyzable silyl groups and inorganic acid (C), forming the above mixture into a film, and hydrolyzing/condensing the hydrolyzable silyl group contained in the mixture formed into the film, to form a three-dimensionally crosslinked silicon-oxygen structure (A). The present invention also provides a fuel cell which incorporates the above proton-conducting membrane.