The present invention is a method of producing a (meth)acrylic acid based water-soluble polymer having a content of groups in neutralized state of not more than 90 mole percent, where the sum total of acid groups and groups in neutralized state contained in the (meth)acrylic acid based water-soluble polymer is taken as 100 mole percent, which comprises a photopolymerization step of polymerizing a monomer component by irradiating near-ultraviolet rays to a reaction mixture, the said reaction mixture comprising the monomer component, a photopolymerization initiator, a chain transfer agent and a polymerization solvent, the said monomer component containing not less than 50 mole percent of a (meth)acrylic acid monomer, with the whole monomer component subjected to polymerization being taken as 100 mole percent, and a concentration of the said monomer component in the reaction mixture at the time of polymerization being 40 to 97% by mass.