The invention improves a sizing effect even on paper containing no internal sizing agent and improves productivity by making solution polymerization in a small amount of an organic solvent. The invention provides a cationic surface sizing agent composed of a copolymer obtained by solution polymerization of (a) 15 to 35% by weight of a tertiary amino group-containing monomer, (b) 30 to 85% by weight of C1 to C4 alkyl(meth)acrylate, and (c) 1 to 50% by weight of styrenes in an organic solvent in the presence of a chain transfer agent. The copolymer may be subjected to cationization treatment with a quaternization agent. As a constituent monomer of the copolymer, 0 to 20% by weight of another copolymerizable vinyl monomer (d) may be contained. The cationic surface sizing agent is applied to paper.