We have investigated the physical properties of (Ba1-xPbx)TiO3 particles affecting the conductivity of the composite solid electrolyte Na4Zr2Si3O12/(Ba,Pb)TiO3. With increasing the lead content x, the dielectric constant decreases, simultaneously the magnitude of the conductivity enhancement in the composite electrolyte increases. We find experimentally that the lead content in (Ba,Pb)TiO3 has an effect on the conductivity enhancement, whereas it has no direct effect on the dielectric constant.