Proposed is a method in which a denitrification treatment that allows an extremely low nitrogen concentration range to be stably and quickly reached is performed, or further this denitrification treatment and a desulfurization treatment are performed within a single treatment, such that denitrification, or denitrification and desulfurization, of molten steel can be efficiently performed. This molten steel denitrification method is a molten steel denitrification treatment in which CaO-and-Al2O3-containing slag 4 is formed on top of molten steel 3 charged in a vessel 1 and an oxygen-containing gas is blown from above the slag, while the slag and the molten steel are brought into contact with each other to remove nitrogen in the molten steel. This method is characterized in that an Al concentration in the molten steel is kept at or higher than a value determined by Formula (1) according to a stirring power density, and that the oxygen-containing gas is blown such that a ratio between a thickness Ls0 of the slag and a depth Ls of a depression in the slag resulting from blowing of the oxygen-containing gas meets Ls/Ls0 ≥ 1: Al concentration (mass%) in molten steel = -0.072 × ln (stirring power density (W/t)) + 0.5822 ···· (1)