To provide a method for manufacturing a low phosphorus steel by dephosphorization smelting with high efficiency.

The dephosphorization method for molten pig iron which is made not to directly contact with top-blown oxygen by intercepting oxygen with slag, when dephosphorization smelting the molten pig iron along with adding flux and top-blowing oxygen, comprises determining whether the molten pig iron contacts with the top-blown oxygen or not, by a dust densitometer installed in an exhaust gas duct, and adjusting a flow rate of top-blown oxygen and/or height of top-blowing lance so that the top-blown oxygen does not contact with the molten pig iron. A dust densitometer installed in the exhaust gas duct determines a melting state of dephosphorization flux added before or just after starting blowing, into the slag, and controls the top-blown oxygen so as not to directly contact with the molten pig iron by being intercepted with the slag after the flux is molt.