To provide a method of water-granulation treatment of copper-refining slag which allows reduction of As eluted from the copper-refining slag more easily.

This water-granulation treatment method uses granulating water of a circulation system and comprises a step of crushing the copper-refining slag with the granulating water, a step of sedimentation-separating suspended solids from the granulating water circulating in the circulation system using a settling tank and extracting a part of the granulating water in the settling tank from the circulation system, and a step of feeding makeup water with an As concentration of 0.01 mg/L or less by an amount for total amount of granulating water flowing in the circulation system to be kept at a steady level, where the extracted amount of granulating water in the settling tank is made to a level so that the As elution amount from the copper-refining slag after the water-granulation treatment becomes 0.01 mg/L or less.