A water-cooling jacket structure 10 for an inspection hole of a flash furnace 1 is arranged at the periphery of a concentrate burner 7 to inspect and clean the inside of the furnace and the concentrate burner 7, and formed in a cylindrical shape by combining a plurality of jacket plates 11, 12, 13, 14 cast internally with cooling tubes 21, 22, 23, 24 for circulating cooling-water, and configured to adjust its cooling capacity and amount of cooling-water by circulating cooling-water to a single or multiple systems of the respective cooling-water systems of each jacket 11, 12, 13, 14 depending on the heat-load of the flash furnace 1.