a gas knife for adjusting a plating thickness by spraying a wiping gas onto a steel sheet passing through a plating bath;

a defect prevention portion installed downstream of the gas knife and cooling the steel sheet by spraying a cooling gas onto the steel sheet;

a moving portion connected to the defect prevention portion; and

a driving portion configured to move the moving portion up and down with the defect prevention portion connected thereto,

wherein the defect prevention portion includes:

a main body having a tubular shape and having a nozzle; and

a supply pipe connected to the main body and supplying the cooling gas to the main body, and

wherein a length of the main body is shorter than a width of the steel sheet such that the cooling gas is sprayed toward a center of the steel sheet in a larger flow amount of the cooling gas than that sprayed towards edges on both sides of the steel sheet, and

wherein the driving portion is configured to move the moving portion and the defect prevention portion connected thereto according to a moving speed of the steel sheet, the width of the steel sheet and a plating amount so that the defect prevention portion sprays the cooling gas towards a position where a comb-patterned surface defect occurs at both edges of the steel sheet, the position varying with the moving speed of the steel sheet, the width of the steel sheet and the plating amount.