PURPOSE: To permit the operation of a dust removing device, satisfying the conditions of phosphorus removing treatment, and contrive the reduction of a gas cost as well as the stabilized operation of the device when exhaust gas dust, generated from a pig iron mixing car in a molten pig preliminary treating and refining process, is treated.

CONSTITUTION: When exhaust gas dust, generated from a treating vessel upon molten pig preliminary treatment, is treated, the operating condition of a dust removing device in an exhaust gas cooling device is preset in accordance with a phosphorus removing treatment condition (ratio of gas oxygen in phosphorus removing agent) to remove the dust.