Method for desulfurizing molten pig iron and desulfurizing agent

Provided are method for desulfurizing molten pig iron and a desulfurizing agent capable of reducing costs required in desulfurization treatment while also obtaining predetermined desulfurization ability without the use of fluorine. A desulfurizing agent (18) is added to molten pig iron (16) when the molten pig iron (16) is stirred and subjected to desulfurization treatment, the desulfurizing agent (18) being obtained by mixing calcium aluminate and quicklime so that the mass-concentration ratio Al2O3/(Al2O3 + CaO) is 0.01-0.20 and the mixing ratio of the calcium aluminate in the desulfurizing agent (18) is greater than 0 mass% and 75 mass% or less.