To efficiently melt iron scrap and to reduce the consumption of electric power, at the time of melting the iron scrap together with molten iron in an arc furnace providing a preheating chamber directly connected with a melting chamber.

In the operating method in the arc furnace 1 provided with the melting chamber 2 and the shaft furnace type preheating chamber 3 directly connected with the melting chamber and introducing exhaust gas generated in the melting chamber, the molten iron is directly charged and also, while continuously or intermittently charging the iron scrap 15 into the preheating chamber so as to hold the state, in which the iron scrap continuously exists in the preheating chamber and the melting chamber, the iron scrap and the molten iron in the melting chamber are heated with the arc 19 to melt the iron scrap. At the point of time when the molten steel 17 is stored at least by one heat quantity in the melting chamber, the molten steel is tapped off in the state, in which the iron scrap continuously exists in the preheating chamber and the melting chamber.