Chloride ion (Cl-) conducting characteristics were investigated in calcium doped lanthanum oxychlorides which show a high stability even in a high humid atmosphere and at an elevated temperature. The Cl- ion conductivity of the oxychloride is approximately 10(-4.5) S.cm(-1) at 700 degreesC and the Cl- ion transference number is almost unity. The development of new types of Cl- ion conducting solid electrolytes with a high durability opens a door to realize a new application for functional materials.