An environmentally friendly flux for molten steel desulfurization includes CaO and Al2O3 so that [CaO]/[Al2O3] is within a range of 1.6 to 3.0, and includes one or more alkali metal oxides of Na2O, K2O, and Li2O, and SiO2 so that [SiO2]/[R2O] is within a range of 0.1 to 3, [R2O] is within a range of 0.5 mass % to 5 mass %, and [SiO2] is within a range of 0.05 mass % to 15 mass % in a case in which the [CaO], the [Al2O3], the [SiO2], and the [R2O] represent the mass % of CaO, the mass % of Al2O3, the mass % of SiO2, and the total amount of the mass % of Na2O, the mass % of K2O, and the mass % of Li2O respectively.