A sizing agent for matrix-resin-reinforcement fiber, a synthetic fiber strand sized therewith, and a fiber-reinforced composite material reinforced by the sized fiber strand. The sizing agent for reinforcement fiber contains a polyamide (A), a carbodiimide group-containing compound (B) and water (C), wherein the polyamide (A) has a melt viscosity ranging from 100 to 15,000 mP·s at 150° C. and the compound (B) has at least two carbodiimide groups per molecule. The polyamide (A) is preferably a water-soluble polyamide.