.[.Disclosed herein is a vinylamine copolymer having the structural units represented by the following formulas (I), (II), (III), (IV) and (V): ##STR1## wherein X.sup..crclbar. represents an anion or hydroxyl ion, R represents a hydrogen atom or a methyl group, M.sup..sym. represents a hydrogen ion or a monovalent cation, in which the molar fraction of the structural unit (I) is 5 to 85% by mole, the molar fraction of the structural unit (II) is 2 to 85% by mole, the molar fraction of the structural unit (III) is 5 to 80% by mole, the molar fraction of the structural unit (IV) is 0 to 40% by mole, all of said molar fractions being based on the vinylamine copolymer and the molar fraction of the structural unit (V) is 0 to 8% by mole which is based on the total content of the structural units (III), (IV) and (V), a flocculating agent and a paper strength increasing agent using the vinylamine copolymer, and a process for producing the vinylamine copolymer..]..Iadd.Disclosed herein is a vinylamine copolymer comprising structural units represented by the following formulas: ##STR2##.Iaddend. wherein X.sup..crclbar. represents an anion or hydroxyl ion, R represents a hydrogen atom or a methyl group, M.sup..sym. represents a hydrogen ion or a monovalent cation, in which the molar fraction of the structural unit (A) is 5 to 85% by mole, the molar fraction of the structural unit (B) is 2 to 85% by mole, the molar fraction of the structural unit (C) is 5 to 80% by mole, all of said molar fractions being based on the vinylamine copolymer, and the molar fraction of the structural unit (D) is 0 to 20% by mole which is based on the total content of the structural units (C) and (D), wherein the total of the molar fractions of all structural units of the vinylamine copolymer is equal to 100%.