A flattening layer is provided between an anode and a solid electrolyte.
The flattening layer has a porosity which is set to be smaller than that
of the anode. Large diameter pores, which are open at an end surface of
the anode on a side facing the solid electrolyte, are filled with the
flattening layer, and projections, which bulge out of the end surface,
are buried therewith to flatten the projections. Therefore, when the
solid electrolyte is formed on the flattening layer, the solid
electrolyte is prevented from any depressions and bulges on which stress
is concentrated.