Solid Electrolytes (SE) for all solid Sodium-Ion Batteries (SIB) have been attracting much attention as an alternative energy storage system, which have the advantage of being extremely safe because it can be charged quickly and is nonflammable. We have synthesized anti-perovskite type Na3OX (X = Br, and I) electrolytes with high purity. It was confirmed that a large crystal strain was introduced by eutecticization, which reduced the activation energy of Na ion conduction and lead to an improvement of the electric conductivity. In our previous work, an ionic conductivity of σ = 1.55x10-7 S/cm for Na3OBr0.6I0.4, and the activation energy was also reduced from 1.14eV to 0.64eV. These values, however, are still poor for a practical application as SE. In this paper, we focused on the Na3OBr1-xIx (x = 0 to 0.4) series and increased the synthesizing time from 3x6 to 3 x 9 hours. An extremely low activation energy of 0.25eV and high ionic conductivity of σ = 2.6x10-3 S/cm at 90oC and have been observed.