Provided is a continuous hot-dip galvanizing apparatus comprising: a vertical annealing furnace having heating, soaking zone, and cooling zones therein; and a hot-dip galvanizing line downstream of the cooling zone. The heating, soaking, and cooling zones each have, in its upper portion, at least one upper hearth roll and, in its lower portion, at least one lower hearth roll. The soaking zone has a first and second humidified gas supply ports to supply a humidified gas having a dew point of 10° C. to 30° C. to the soaking zone. The first and second humidified gas supply ports are 1.0 m to 5.0 m lower than the center of the lower and upper hearth rolls, respectively, and overlap the steel sheet. The first humidified gas supply port is provided only for an ascending pass and the second humidified gas supply port is provided only for a descending pass.