Abstract: A method for producing a lithium lanthanum zirconium oxide (LLZO)-based ceramic according to an embodiment of the present invention comprises a sintering operation of sintering an oxide sheet that includes LLZO powder particles including the elements lithium, lanthanum, and zirconium. The sintering operation may include a first sintering operation of sintering at a temperature of T1 and a second sintering operation of sintering at a temperature of T2 lower than T1. An LLZO-based ceramic according to another embodiment of the present invention has a relative density of at least 88% and comprises one or more crystal grains having a size of 1-10 ?m, wherein the standard deviation of the grain size is 80% or less of the average grain size, and the grains may have a cubic crystal structure.