Provided is an integrated system for manufacturing steel including an iron making apparatus and a steel making apparatus. The steel making apparatus produces molten steel from molten iron and agglomerated reduced iron received from the iron making apparatus. The iron making apparatus includes a first iron ore reducing device, first and second agglomerating devices, and a smelting furnace. The fine iron ore reducing device includes first and second fluidized reduction furnace apparatuses respectively including at least one fluidized reduction furnace for reducing fine iron ore. The first and second agglomerating devices agglomerates reduced fine iron ore received from the first and second fluidized reduction furnace apparatuses, respectively. The smelting furnace produces molten iron by melting agglomerated reduced iron received from the first agglomerating device.