A sintered bulk Li7La3Zr2O12 (LLZ) pellet was prepared in Ar flow. The Li ion conductivity of LLZ pellet sintered in Ar flow was lower than that sintered in air due to mainly high grain boundary resistance, indicating that air promotes sintering of LLZ. On the other hand, stability against molten Li metal was not affected by sintering atmosphere. LiCoO2 was prepared by a sol–gel method and no impurity phase was observed by XRD identification. All-solid-state battery with Li/LLZ/LiCoO2 configuration was constructed and operated successfully.