An alcohol solution of a metal alkoxide is mixed with a H.sub.3PO.sub.4 mixture solution of water and alcohol to hydrolyze the metal alkoxide and effect crosslinking with H.sub.3PO.sub.4. Accordingly, a solid is produced composed of a composite oxide glass (proton conductor) containing constituent elements of P and a metal element originating from the metal alkoxide and having hydroxyl groups at terminals. The metal alkoxide is selected so that the metal alkoxide contains constituent elements of metal elements each of which is designated by a chemical shift within -25 ppm. The chemical shift is a difference between a value at which a peak of orthophosphoric acid appears when a silicon oxide-phosphate glass, which is a composite oxide of phosphorus oxide and silicon oxide, is analyzed by P.sup.31--MASNMR and a value at which a maximum peak appears when the composite oxide glass is analyzed by P.sup.31--MASNMR.