PURPOSE: To obtain a uniform alloyed galvanized steel plate by providing a burner for heating a steel plate in an alloying treating furnace in such a way that the edge parts of the steel plate can be positively heated for each of the width of the product plate to be produced and making the fuel injection of the burner adjustable according to the width of the product plate to be produced.

CONSTITUTION: Burners are arranged in the positions of the central part O and edge parts B, (b) in the transverse direction of a steel plate in the case of 600mm width of the min. product plate to be produced and in the positions of O and A, (a), C, (c) in the case of 800mm width, so that the edge parts of the various widths of the products to be produced can be positively heated up to the max. width of the product to be produced. Plural sets of the burners are disposed in a longitudinal direction. Valves for adjusting injection are provided to the burners, and the width of the plate is detected by the width gauge provided in the inlet side of an alloying furnace. The solenoid valves of nozzles are operated in accordance with the detected values, whereby the fuel injection conforming to the width of the plate is accomplished.