A method of operating an electric furnace is provided. A method of operating an electric furnace includes melting a first iron source in a first melting furnace in which a first electrode unit is disposed, preheating a second iron source in a second melting furnace in which a second electrode unit is disposed, and melting the second iron source in the second melting furnace, wherein the first melting furnace and the second melting furnace share an internal space, in the preheating of the second iron source, the second electrode unit emits a first gas, and in the melting of the second iron source, the second electrode unit emits a second gas that is different from the first gas.