PURPOSE: To stabilize the distribution of gas flow in a furnace by perfectly preventing segregation in an upper and a lower bunkers, making the variation of discharged grain diameters into the furnace from the lower bunker with time the flat pattern, reducing influence of the variations of the grain size and the charging rate as less as possible and improving the adjusting precision of the charging schedule.

CONSTITUTION: Double-tube segregation preventing parts 3 separated between an inner cylinder 1 and an outer cylinder 2 are arranged in the upper and the lower bunkers 4, 5, and raw material supplied from a charging belt conveyor on the furnace is not reclassified (dispersion of the grain diameters) in the bunker, but is charged in the condition of no segregation of the grain size from the upper bunker 4 to the lower bunker 5. Then the variation of the discharged grain diameters with time at the time of discharging into the furnace is made to be 'the flat pattern-no change of the grain diameter' and the adjusting precision of the charging schedule (a small variation of the grain size and the charging rate) is improved and the distribution of gas flow is stabilized.