PURPOSE: To obtain a sizing agent for a spray sizing, not clogging in a nozzle, without showing a fogging with the size and not generating rust by copolymerizing a polyvinyl alcohol or its derivative with an α,β-monoethyleneic monomer in a specific weight ratio.

CONSTITUTION: This sizing agent consisting of a polyvinyl alcohol(PVA) or its derivative, a graft polymer of PVA or its derivative with an α,β- monoethylenic monomer and a polymer of the α,β-monoethylenic monomer is obtained by blending 50-100 pt.wt. α,β-monoethylenic monomer [e.g. ; vinyl acetate and (meth)acrylic acid] with 100 pts.wt. aqueous solution of a PVA or its derivative (e.g.; a PVA modified by copolymerization with an olefin) and copolymerizing. The sizing agent does not generate a clogging in a spraying nozzle, has an excellent sizing effect to a synthetic fiber, shows a good finished touch feeling and does not putrefy.