When a portion for measuring the plating adhesion amount reaches an upstream side position, plating adhesion amount estimation values are calculated by using a plating adhesion amount estimation expression at positions away from a position that faces the distance sensors, that is, the upstream side position, by strip-width direction distances, of the surfaces of the steel strip. When the portion for measuring the plating adhesion amount reaches a downstream side position, the strip-width direction distances of the plating adhesion amount meters are matched to the strip-width direction distances, and the plating adhesion amount actual measurement values are obtained. The plating adhesion amount estimation expression is corrected on the basis of the differences between the plating adhesion amount estimation values and the plating adhesion amount actual measurement values. Accordingly, the control accuracy of the plating adhesion amount is improved.