PURPOSE: To prevent dust from attaching, by substituting part of air introduced from an opening on the suction side of a cooling drum used in a foundry with a hot air to keep the temperature of a mixed discharge gas, which is sent to a dust collector, higher than the dew point.

CONSTITUTION: A heating air at a temperature of about 70°C is guided to an opening 2 on the suction side of a cooling drum 1 by means of a hot air fan 10 so that the transfer air introduced from the opening 2 on the suction side is increased without increasing the quantity of said air. Dust and steam generated in the cooling drum 1 are sent, together with the hot air, from the delivery side of the drum 1 to a dust collector 8, so that the temperature of the mixed discharge gas is held higher than the dew point. In this manner, dew condensation in the passage between the deliver port 5 of the cooling drum 1 and the dust collector 8 can be prevented without raising the performance or the running cost of an exhaust fan 9 of the dust collector 8, eliminating thereby dust clogging and degradation of the performance of the dust collector.