To provide a cooling method of desiliconization slag for effectively suppressing elution of fluorine from a slag product by treating a desiliconization slag containing fluorine at a low cost.

When cooling the siliconization slag discharged from a reaction container by spraying in desiliconization of hot metal, the spraying is performed to the desiliconization slag with spray density of 0.5 t/Hm2 or less. Further, the desiliconization slag cooled by spraying in the conditions is worked into a civil engineering material. By reducing the spray density, inflow of water to the inner part of a high-temperature slag mass is suppressed, therefore, production of a soluble fluoride is suppressed and, as the result, the elution amount of fluorine from the slag product is effectively reduced.