Provided are: a flame retardant composition which can impart excellent flame retardancy to a synthetic resin even when added in a small amount; and a flame-retardant synthetic resin composition which contains the flame retardant composition and a resin and exhibits excellent flame retardancy. The flame retardant composition contains components (A) and (B), wherein (A) is a phosphate-based flame retardant, and (B) is a hindered amine compound having a structure represented by Chemical Formula (1) in the structure. In Chemical Formula (1), * represents a bond, and one or more of the structures represented by Chemical Formula (1) optionally exist in the compound.