An object of the present invention is to provide a process for easily producing a water-soluble polymer which has a reduced residual monomer amount and has excellent various flocculating performances when used as a flocculant, the process being capable of grafting a high molecular weight polymer onto starch. A process for producing a water-soluble polymer is provided, which comprises polymerizing water-soluble radical-polymerizable monomers comprising a cationic radical-polymerizable monomer as an essential component in a presence of a polysaccharide, an azo polymerization initiator, and a hydrogen-abstracting agent. A polymer flocculant is also provided, which comprises a water-soluble polymer obtained by the production process.