PURPOSE: To obtain the primary sizing agent for glass fiber small in migration by incorporating a water-soluble epoxy resin into the sizing agent, and associating the water-soluble epoxy resin within a specified pH range thereby forming particles having a diameter of a prescribed length.

CONSTITUTION: About 0.5-2mol amine such as diethanolamine, 1mol epoxy resin such as bisphenol A are subjected to an addition reaction to prepare water-soluble epoxy resin. This water-soluble epoxy resin, about 0.1-1wt.% each of a surface treating agent such as aminosilane, a lubricant component such as butyl stearate, an antistatic agent and acetic acid per the resin are added to and dissolved into water to prepare the primary sizing agent for glass fiber containing 1-2wt.% water-soluble epoxy resin and forming particles having 0.5-50μm diameter by associating the water-soluble epoxy resin at pH 5.5-7.5 and small in migration. This primary sizing agent is applied onto the glass fiber by touch roller system, etc., then subjected to washing treatment to obtain the glass fiber having about 0.1-1wt.% sizing agent coating ratio.