At the beginning of a dezincification plant used in a dezincification step in a hydrometallurgical method for nickel, a decrease throughout the dezincification plant is controlled to prevent a decrease in production volume and a cake layer is formed on a filter cloth provided to a filter device inside the dezincification plant. At the beginning of dezincification plant, a slurry containing a formed zinc sulfide is supplied to a filter for filtration and separation, an adjustment is performed in which the flow rate of the slurry is increased to reach a target flow rate in a time T2 which satisfies the following relational expression 3×T1≦T2≦5×T1, where T1 represents the time between starting a slurry supply and attaining the target flow rate in the case of transferring the slurry at the maximum liquid transfer capacity of a pump configured to transfer the slurry.