PURPOSE: To eliminate the positive pressure in an air tunnel, prevent the blowing out of granular slag and slag wool, suck the cold air of the outside and improve the cooling effect for the granular slag by providing an air vent port in the ceiling part of the air tunnel for blowing out of the granular slag in producing the granular slag by blowing molten slag.

CONSTITUTION: Molten slag 2 discharged from a blast furnace is flowed down from a tundish 1 through a down-flow trough 5, and is blown into an air tunnel 6 by the high pressure through a nozzle 4, whereby granular slag as aggregate for concrete and subbase course materials is produced. In this case, slag wool is by- produced in the air tunnel 6, and the pressure in the air tunnel rises to a positive pressure, by which granular slag and slag wool are blown through the clearances of the air tunnel and this degrades the working environment and lowers the recovery efficiency of the granular slag. In order to prevent this, an air vent port 14 is provided to the top part 6b of the air tunnel 6, and an air releasing cylinder is mounted thereto. The positive pressure in the air tunnel 6 lowers to atmospheric pressure and there is no more blowing out from the air tunnel 6. Conversely, the cold air of the outside of the air tunnel 6 is sucked, whereby the cooling effect of the red hot granular slag is improved.