To effectively use a lime content in a reducing slag as a slag-making material, by decreasing a sulfur content in the reducing slag.

This treatment method includes blowing steam into the reducing slag in a treatment pit in a treatment facility to make the reducing slag contact with the steam. Thereby, calcium sulfide in the reducing slag is hydrolyzed, and calcium hydroxide and hydrogen sulphide are produced. Then, the hydrogen sulphide is released to the atmosphere. In this way, the sulfur content in the reducing slag is eliminated from the reducing slag in a form of hydrogen sulphide. The reducing slag after having been treated with steam is reused as the slag-making material for reduction refining in a steelmaking process with the use of an electric furnace.