A sizing agent for matrix-resin-reinforcement fibers that simultaneously attains excellent cohesion and abrasion resistance of sized fibers, uniform size application on fiber surface and good bonding between sized fiber and a matrix resin; a synthetic fiber strand sized therewith; and a fiber-reinforced composite material reinforced by the sized fiber strand. The sizing agent 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 mPa·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.