To cool the inside of a furnace in a short time by using a cooling system having a simple structure.

A cylindrical guide 2 is arranged coaxially with a shaft S of a cooling fan 1 on the front side of the cooling fan 1, and straightness of a cooling air flow forcibly sent from the cooling fan 1 is secured by the guide 2. A spray nozzle 3 is arranged in the shaft center in the guide 2. The spray nozzle 3 is arranged coaxially with the shaft of the cooling fan 1, and is supported by the guide 2 via three frames. A tip part of the spray nozzle 3 is a conical shape, and a plurality of spray holes 3a are opening in the conical tip part. Mist-like cooling water is sprayed from the plurality of spray holes 3a, and the mist-like cooling water can be included in the cooling air.