To provide a method for dephosphorizing molten pig iron by concurrently using a method of top-blowing oxygen, which has improved dephosphorization efficiency without evolving CO gas due to a decarbonizing reaction and expanding a facility for treating emitted dust.

(1) The method for dephosphorizing the molten pig iron with the use of a device for mechanically agitating the molten pig iron by rotating a blade for agitation includes adjusting a height of a lance and a flow rate of gaseous oxygen while top-blowing oxygen by using the lance from the upper side of slag surface on molten pig iron, into such a range as not requiring the increase of the capacity of a dust collector due to the increase in an amount of dust originating in the decarbonizing reaction. Then, the method promotes the dephosphorization reaction without increasing a power for mechanical agitation. (2) In the method (1), it is preferable to top-blow gaseous oxygen so that a calculated value of a recess depth formed on the molten pig iron by a stream of the gaseous oxygen is in a range of 30 to 40 mm.