To elucidate the chemical and structural changes of sulfide solid electrolyte during heat treatment, in situ outgas (TPD-MS), Raman, and XRD analyses were applied for a 70Li(2)S-30P(2)S(5) glass sample. A sulfide solid electrolyte was constructed from the PS43- and P2S74- anions, together with a small amount of P2S64- anions. In the course of the structural change from glass to glass-ceramic through heat treatment, the fraction of PS43- anion decreased and P2S74- anion increased at 210 degrees C. Sulfur compounds were detected as outgas at about 240 degrees C, which suggests thermal decomposition of PS43- anion to produce P2S74- and sulfur compounds as by-product. The thermal treatment temperature should be higher than 240 degrees C to eliminate sulfur compounds generated as the reaction by-product. In the same temperature region, crystallization of Li7P3S11 was also detected using in situ XRD. The ionic conductivity increased from 1.0 x 10(-4) S cm(-1) to 8.4 x 10(-4) S cm(-1) after heat treatment at 270 degrees C. Therefore, the partial compositional change of PS43- to P2S74- anions and crystallization of Li7P3S11 are key factors producing higher ionic conductivity. By combining these experimental techniques implemented in this study, it is possible to detect the temperature at which the chemical and structural changes proceed. One can also ascertain the suitable heat treatment temperature for production of by-product free and high-quality sulfide based solid electrolyte.