A lanthanum oxide (La.sub.2O.sub.3) powder, a germanium oxide (GeO.sub.2)
powder, and a strontium carbonate (SrCO.sub.3) powder are mixed in a
ratio so that a composition of the obtained composite oxide
La.sub.lX.sub.m(AO.sub.4).sub.6-n(ZO.sub.4).sub.nO.sub.p satisfies
8.ltoreq.l+m<10, 0.ltoreq.m.ltoreq.2, 0.ltoreq.n.ltoreq.2 and
0.ltoreq.p.ltoreq.2. Thenafter, the materials are formed and sintered to
prepare an oxide ion conductor. The crystalline structure of
La.sub.lX.sub.m(AO.sub.4).sub.6-n(ZO.sub.4).sub.nO.sub.p belongs to the
apatite type structure. The conduction of oxide ion occurs when O.sup.2-
14 occupying the 2a site of the apatite type structure moves along the
c-axis direction.
