A new type of chloride ion (Cl-) conducting solid electrolyte was realized for calcium-doped lanthanum oxychlorides, which hold a high stability even in a high humid and also at elevated temperatures. The Cl- ion conductivity of the oxychloride is around 7.1 x 10(-4) S cm(-1) at 700 degreesC and the Cl- ion transference number is estimated to be almost unity. The present Cl- ion conducting solid electrolyte holding a high durability even in elevated temperatures is greatly expected to be applied for functional materials such as Cl-2 gas sensors, etc.