Proposed is a method for efficiently manufacturing low-phosphorus molten steel by use of a steelmaking electric furnace, in which slag resulting from melting a solid iron source is effectively separated from molten steel, and thus a unit consumption of lime required to reduce a phosphorus content in the molten steel is reduced. The method for manufacturing low-phosphorus molten steel includes: a first step of charging a solid iron source and an optional molten iron source and melting and heating these raw materials by using electric energy; a second step of partly or entirely removing slag generated during the melting; a third step of, after the second step, performing dephosphorization by adding dephosphorization flux; and a fourth step of tapping low-phosphorus molten steel thus refined, and in the method, a slag composition ratio C/(S + A) of the slag to be removed in the second step is adjusted to be in a range of not less than 0.25 and not more than 0.70. Here, the slag composition ratio C/(S + A) is determined by dividing a CaO concentration (C) by a sum of an SiO 2 concentration (S) and an Al 2 O 3 concentration (A) on a mass basis in the slag.