To develop a rosin type emulsion sizing agent having high emulsion stability and developing excellent sizing properties in a papermaking system.

The polymeric emulsifying dispersant comprises a partially or perfectly saponified product of a copolymer obtained by reacting isoprene sulfonic acids and a hydrophobic monomer or a copolymer obtained by further reacting unsaturated carboxylic acids with them. Each of the isoprene sulfonic acids is a conjugated diene type sulfonic acid-containing monomer and more excellent in copolymerizability than styrene sulfonic acids and can more uniformly introduce a sulfonic acid group into the copolymer. The glass transition point of the copolymer using isoprene sulfonic acids is made lower than that of the copolymer using styrene sulfonic acids and the sizing agent deposited on pulp fibers is effectively melted in a drying process and, therefore, the polymeric emulsifying dispersant is excellent in emulsion stability and sizing properties.