To provide a sizing agent having excellent dividing property after sizing, reduced in damage of a film by friction and capable of providing a warp having slight fluff and a glass fiber yarn for warp to which the sizing agent is attached in order to prepare a glass fiber woven fabric having slight fluff.

The film-forming agent and the warp sizing agent for glass fiber woven fabrics are each obtained by formulating a polyvinyl alcohol with a hydroxyalkylated starch and/or an acetylated starch and the decomposed product of a cationized starch or the decomposed product of a cationized hydroxyalkylated starch or by formulating the polyvinyl alcohol with the decomposed product of the cationized starch and/or the decomposed product of the cationized hydroxyalkylated starch. The warp for glass fiber woven fabrics is obtained by attaching the sizing agent to the glass fiber yarn warp.