PURPOSE: To prevent the generation of defectives near the joint of preceding and succeeding materials by controlling hot wind temps. and the rate of blowing of said wind ingeniously and changing the quantity of charging heat in a short time in the stage of changing plate thicknesses.

CONSTITUTION: In the stage of changing a strip from a thin to thick strip in a continuous painting and baking furnace, a continuous heating and annealing furnace, etc., the temp. for blowing hot wind is beforehand reduced in accordance with the increase in the rate of blowing of the hot wind and the hot wind temp. is increased quickly at the same instant of changing the thickness. After the change of the thickness, the how wind temp. is reduced in accordance with a gradual increase in the rate of blowing. In the stage of changing the strip from a thick to thin strip, the hot wind temp. is beforehand increased in accordance with a gradual decrease in the rate of blowing of the hot wind, and at the same instant of changing the thickness, the hot wind temp. is lowered quickly. After the change of the thickness, the hot wind temp. is increased with a gradual decrease in the rate of the blowing. In this way, the quantity of the charging heat is changed in a short time with gentle fluctuations in the rate of the blowing at the changing of the thickness and the generation of defectives is prevented.