temperature is 450 u00b0 c or more are decomposed herein it is known that when the solid electrolyte used in the present invention is utilized in a closed system the solid electrolyte is not decomposed at a temperature of up to about 590 u00b0 c and is melted at about 590 u00b0 c therefore it can be considered that the solid electrolyte should be fired at lower than 590 u00b0 c however the inventors of this application found that when the solid electrolyte is used in an open system for example when the solid electrolyte is used as a material of solid state lithium secondary batteries the solid electrolyte is decomposed at about 450 u00b0 c the solid electrolyte in a closed system exhibits a behavior different from that in an open system in such a manner because of the reason described below in a closed system when the solid electrolyte is sublimated with a temperature increase the pressure in the system is increased and therefore the solid electrolyte is melted without being decomposed as described above in contrast in an open system even if the solid electrolyte is sublimated with a temperature increase the pressure in the system is not increased and therefore the solid electrolyte is decomposed as described above accordingly the firing temperature needs to be controlled to 400 u00b0 c or lower in the present invention the firing temperature of the solid electrolyte is controlled to 100 u00b0 c or higher this is because if the firing temperature is excessively low sintering effects are not sufficiently produced and lithium ion conductivity is not sufficiently improved it is also recognized that the discharge capacities of the invention cells a b 2 and b 3 are larger than those of.