PURPOSE: To uniformize cooling in the direction of a flange width and to cool the flange so that it is not deflected by deciding the density of cooling water from the cooling ratio of a swell part of cooling water to a spray cooling zone.

CONSTITUTION: The swell height (h) of the swell part 5 of the cooling water on a flange surface is calculated in advance by an oblique angle α, a spread angle γ and a twist angle β of a spray nozzle for cooling water and a cooling ratio of the swell part to the spray cooling zone is calculated from a relation between an allowable quantity of deflection in the length direction of a product and the swell height (h) of the cooling water. The cooling speed of the swell part is determined by the ratio of the respective coefficients of thermal conductivity because it originals in a quantity of spray cooling water. Then, the density of spray cooling water is determined from this cooling ratio to cool the flange 4. In this way, cooling uniform in the width direction of the flange 4 and free from deflection is obtained.