PURPOSE: To satisfactorily size a sheet formable cellulosic fibrous material at a pH value of a neutral region to an alkaline one by adding both a sizing material and a specific cationic material to the above fibrous material at a wet-end stage.

CONSTITUTION: This method is to add both a sizing material and a cationic material to a sheet formable cellulosic fibrous material at a wet-end stage. The cationic material is a powdered water-insoluble aluminum compound on the surface of which is present a substance [e.g. a poly(t-amine) or the like] which is cationic at pH 6-10.