To obtain a paper strengthening agent having high freeness, yield and paper strengthening effect even with the high compounding ratio of a water- soluble/dispersible natural polymer such as starch.

This paper strengthening agent is a combination of a cationic polymer with an aqueous grafted natural polymer solution 20-50 wt.% in solid content obtained by graft copolymerization of vinyl monomers containing an anionic monomer as the essential component in an aqueous solution of the water-soluble/dispersible natural polymer.