Manganese is an important alloying element for the production of high-grade steel. In Japan, the total amount of Mn disposed of annually in steelmaking slag is close to the amount of imported Mn. Therefore, steelmaking slag can be considered as a Mn resource. Although ferromanganese alloys can be produced by the reduction of steelmaking slag, P content in the product is too high and Mn content is too low, preventing the use of these alloys for the steelmaking process. The separation of P from Mn and increase in the Mn/Fe ratio is thermodynamically possible via the formation of liquid sulfide (matte). High-purity ferromanganese alloys can be obtained by the desulfurization (oxidation) of the formed sulfide. A fundamental experiment to clarify the distributions of Fe, Mn, Ca, and P between matte and steelmaking slag was conducted, and it was found that P was not distributed in matte. Moreover, the separation of P from Mn was confirmed.