In manufacturing of a carboxyl group-containing water-soluble polymer by a precipitation polymerization method, a method which can improve production efficiency with increasing practicality continuously adds a monomer solution wherein a monomer component containing an α, β-unsaturated carboxylic acid is dissolved in an inert solvent in a concentration of 20 to 50% by volume to an inert solvent charged in a reaction vessel. In this case, the amount of the inert solvent charged in the reaction vessel is set so that the concentration of the monomer component in the total amount summed up with the monomer solution is 10 to 24% by volume, and the time period until completion of continuous addition of the total amount of the monomer solution is set so that the conversion of the monomer component at the time of completion of addition of the total amount of the monomer solution is 60% or more. The monomer component may contain a polymerizable compound having two or more ethylenically unsaturated groups.