Abstract: In order to preheat pieces of iron scrap of various sizes and shapes highly effectively while avoiding the fusion of the pieces of iron scrap, a rotary kiln type preheating furnace and a shaft type preheating furnace are arranged in parallel with each other in the front stage of a melting furnace in which iron scrap is melted, and preheated iron scrap is charged from both preheating furnaces into the melting furnace. A damper is provided between the shaft type preheating furnace and the melting furnace, so that exhaust gas discharged from the melting furnace is prevented from directly flowing into the shaft type preheating furnace and introduced into the rotary kiln type preheating furnace. Exhaust gas which has passed through the rotary kiln type preheating furnace is introduced into the shaft type preheating furnace preferably from an upper portion and discharged from a lower portion.