Sulfide solid electrolytes with cubic Na3PS4 phase has relatively high sodium ion conductivity of over 10−4 S cm−1 at room temperature, and all-solid-state sodium batteries Na–Sn/TiS2 with the electrolyte operated as a secondary battery at room temperature. To improve battery performance, conductivity enhancement of sulfide electrolytes is important. In this study, we have succeeded in enhancing conductivity by optimizing preparation conditions of Na3PS4 glass–ceramic electrolytes. By use of crystalline Na2S with high purity of 99.1%, cubic Na3PS4 crystals were directly precipitated by ball milling process at the composition of 75Na2S·25P2S5 (mol%). The glass–ceramic electrolyte prepared by milling for 1.5 h and consecutive heat treatment at 270 °C for 1 h showed the highest conductivity of 4.6 × 10−4 S cm−1, which is twice as high as the conductivity of the cubic Na3PS4 glass–ceramic prepared in a previous report. All-solid-state Na–Sn/NaCrO2 cells with the newly prepared electrolyte exhibited charge–discharge cycles at room temperature and kept about 60 mAh per gram of NaCrO2 for 15 cycles.