A manufacturing method of an electrode assembly capable of easily manufacturing a configuration in which an electrolyte and an active material are bonded to each other is provided.A step of supplying, solidifying, and crystallizing a solid electrolyte 22 including Li2+XC1−XBXO3 (X represents a real number equal to or greater than 0 and smaller than 1), so as to be in contact with an active material aggregate 12 including a communication hole 14 between active material particles 13, is included. In a case where the solid electrolyte 22 is melted, the solid electrolyte 22 is heated in a range of 650 degrees to 900 degrees.