Disclosed is an ionic conductor comprising a metal composite oxide characterized by comprising oxygen defects and metal defects in a cryolite lattice. An electrochemical device comprising the ionic conductor is also disclosed. The metal composite oxide has an improved ion conductivity, because formation of an open space within a lattice is ensured by the defects of metal ion sites in the lattice. Therefore, the metal composite oxide is useful for an ionic conductor or an electrochemical device requiring ionic conductivity.