To minimize the warpage of a steel sheet and to minimize the variations in the coating weight in the transverse direction of the steel sheet by detecting the shape of the steel sheet to be plated by using a laser, determining the warpage of the steel sheet from the detected value thereof, comparing the value with a model and moving a sink roll in a horizontal direction.

The steel sheet 15 is irradiated with the spot laser beam through a glass flat plate 17 from a semiconductor laser 18 and the image thereof is picked up by a CCD camera form a diagonal direction. This image pickup signal is digitalized and the digital value is binarized by the suitable threshold. The light from the one semiconductor laser 18 forms a spot image on the front and rear of the glass 17 and the front surface of the steel sheet 15. The shape of the steel sheet 15 is determined by a shape computation device 13 from the position of the steel sheet 15 corresponding to the respective spot laser beams and the warpage of the steel sheet 15 is computed. The relation between the moving quantity in the horizontal direction of the sink roll and the warpage of the steel sheet 15 is previously determined and is modeled. The actually measured warpage and the position of the sink roll of the moded are compared. The sink roll is moved in the horizontal direction according to the result thereof, by which the warpage of the steel sheet is minimized.