An electrolyte is single crystal composed of La.sub.9.33Si.sub.6O.sub.26 in which the crystal is oriented in the c-axis, and the c-axis is in the thickness direction of the electrolyte. In the electrolyte, the oxide ion is preferentially migrated in the thickness direction. That is, the oxide ion conduction exhibits anisotropy. Isotropic conductive layers composed of YDC, in which the oxide ion conduction exhibits isotropy and the oxide ion conductivity is lower than that of the electrolyte, are provided between the electrolyte and an anode and a cathode.