Provided is a powdery paper-strengthening agent comprising an amphoteric (meth)acrylamide-based polymer (A) having a weight-average molecular weight of 1,000,000 to 7,000,000, wherein the amphoteric (meth)acrylamide-based polymer (A) comprises, as constituent monomers, (meth)acrylamide which is an (a1) component, a cationic unsaturated monomer which is an (a2) component, an anionic unsaturated monomer which is an (a3) component, and a crosslinkable unsaturated monomer which is an (a4) component, and wherein an aqueous solution, in which the amphoteric (meth)acrylamide-based polymer (A) is dissolved in a calcium chloride aqueous solution having an electrical conductivity of 4 mS/cm at 25° C. so as to be 1% by weight, has a maximum turbidity of 10 to 2500 NTU at pH 3 to 9.