PURPOSE: To obtain safely and economically an aq. emulsion of high-concentration rosin, which is excellent as a sizing agent for paper making, by incorporating an ultrafiltration operation into a concentration stage for an aq. emulsion of a low-concentration rosin material.

CONSTITUTION: In the production of an aq. emulsion of a high-concentration rosin material from an aq. emulsion of a low-concentration rosin material, an ultrafiltration operation is incorporated into a concentration stage without carrying out a physical or mechanical operation which causes the breakage of emulsion particles and promotes it. The term "rosin material" used includes not only reinforced rosin but also a mixture of rosin and a petroleum resin or a hydrocarbon resin. Said ultrafiltration is a filtration method in which pressure is applied to a solution to be filtered, high-molecular materials and colloidal materials are inhibited by a semipermeable membrance, and water and low-molecular materials contained therein are passed through numberless pores present on the surface of the semipermeable membrane. Said low concentration means concentration lower than 40wt%. Said high concentration means concentration higher than 40wt%.