PURPOSE: To obtain the subject water-dispersible type rosin sizing agent, containing a rosin substance and a specific dispersing agent, excellent in sizing effects and storage stability with hardly any foaming properties and useful for forming sheets, etc., of paper from cellulosic fiber.

CONSTITUTION: The objective rosin sizing agent is obtained by using at least one dispersing agent selected from (A) compounds expressed by formulas I and II [R1 and R2 are H, 8-22C alkyl or alkenyl; M is alkali metal or ammonium; R3 is 1-5C alkyl; m is 2-10; n is 2-12; (m+n) is 8-18], (B) castor oil expressed by formula III (AO is 2-4C alkylene oxide; p is 3-30) and (C) alkali metallic salts or ammonium salt of polypropylene glycol sulfate having 500-4000 molecular weight in a water-dispersible type rosin sizing agent containing a rosin substance such as wood rosin and the dispersing agent. Furthermore, the aforementioned dispersing agent is added in an amount of preferably 1-5wt.% expressed in terms of the solid content based on the rosin substance.