Disclosed is a method of coating a solid electrolyte of a solid-state battery. The method includes putting an electrode active material and a sulfide-based solid electrolyte into a rotating mixer, operating the rotating mixer to rotate in a first direction while the rotating mixer revolves in a second direction, and coating a surface of the electrode active material with the sulfide-based solid electrolyte in the rotating mixer during the operation of the rotating mixer, to form a solid electrolyte layer on the surface of the electrode active material.