PURPOSE: To secure the ventilation in a moving bed and to improve production efficiency by specifying the void ratio of the fine ore packed in a transport pipe at the time of separating the fine ore discharged from the recirculating fluidized bed from a gas and returning the fine ore to the fluidized bed.

CONSTITUTION: The fine ore supplied from a raw material inlet 14 is fluidized by a reducing gas 13 in a riser 10, reduced, and then introduced into a cyclone 11 along with the high-speed gas. The fine ore grains alone are introduced into a downcomer 12, and returned to the fluidized bed. A part of the grains are recovered from an outlet 15. In this operation, the sorting and screening of the raw ore and the mixing of the grains having different physical properties (apparent density, shape, bulk density, etc.) are previously carried out, and the fine ore grains exhibiting ≥0.40 bulk density in the downcomer 12 are used. As a result, the circulation of grains in the recirculating fluidized bed is stabilized, and the ore is smoothly reduced.