PURPOSE: To prevent dew condensation on the surface of an Al slab by coating the slab with a liq. which is not burned off by heating up to about 100°C and is burned off by heating to ≥ about 100°C and by heating the coated slab in a heating furnace.

CONSTITUTION: When an Al slab is planed, charged into a heating furnace, and homogenized or soaked before carrying out hot rolling, the slab is coated with a liq. which is not burned off by heating up to about 100°C in the furnace and is burned off by heating to ≥ about 100°C. The liq. is a soln. of acrylic resin or epoxy resin, rust preventing oil, silicone oil, glycerol or the like. Thus, even if the slab is heated to about 500W600°C, the deterioration of the quality due to dew condensation can be prevented because the state with the formed coat on the planed surface of the slab is maintained until the slab reaches a temp. at which dew condensation is caused.