To provide a sizing agent for a glass fiber, capable of keeping the ability of a curable resin for impregnating the glass fiber at a high level, capa ble of improving the rigidity of a SMC sheet and further capable of improving the water resistance of the obtained glass fiber-reinforced resin.

This sizing agent for the glass fiber is characterized in that the sizing agent comprises an aluminum compound represented by general formula (1) [wherein, R1 and R2 are each an alkyl group; and R3 is an alkyl group or an alkoxy group], a silane-based coupling agent, a film-forming resin, an emulsifier and water.