As a positive electrode active material of a secondary battery, a lithium-manganese composite oxide containing lithium, manganese, and an element represented by M, and oxygen is used, and the lithium-manganese composite oxide is covered with reduced graphene oxide. An active material layer including the active material, graphene oxide, a conductive additive, and a binder is formed and soaked in alcohol, and then heat treatment is performed, whereby an electrode with reduced graphene oxide is fabricated.