We investigated the development of a novel lithium garnet-type oxide electrolyte, which is co-sinterable with a metal oxide cathode, for practical all-solid-state lithium ion batteries using metal oxide electrolytes. The sintering temperature of Li6.8(La2.95,Ca0.05)(Zr1.75,Nb0.25)O12 was found to significantly lower to 790 °C by addition of Li3BO3 and Al2O3, due to simultaneous interdiffusion of Al and Ca elements between garnet-type oxide and additives. The sintered electrolyte exhibited high lithium ion conductivity of 0.36 mS cm−1 at 25 °C despite of the low sintering temperature. An all-solid-state lithium ion battery was successfully prepared by co-sintering of the electrolyte and LiCoO2 (cathode), followed by coating of Li metal (anode), and confirmed to function well as a secondary battery with charge and discharge capacities of 98 and 78 mAh g−1, respectively. These results opened the potential for fabrication of all-solid-state lithium ion batteries by a simple and well-established co-sintering process.