Disclosed is a 12CaO.7Al.sub.2O.sub.3 compound, a 12SrO.7Al.sub.2O.sub.3 compound, or a mixed crystal compound of 12CaO.7Al.sub.2O.sub.3 and 12SrO.7Al.sub.2O.sub.3, which contains a negative hydrogen ion (H.sup.-, H.sup.2-, H.sub.2.sup.- at a concentration of 1.times.10.sup.18 cm.sup.-3 or more. A negative hydrogen ion comprising a primary component of a hydride ion is incorporated into C12A7 (12CaO.7Al.sub.2O.sub.3), so that a function of being converted from an insulative material to an electrically conductive material in a sustained manner by means of irradiation with light can be exhibited even in the normal atmosphere at a room temperature. The present invention also provides a solid electrolyte capable of conducting a negative hydrogen ion, and means for releasing a hydride ion from the inside of a solid into a gaseous phase using an electric field.