The present invention provides a composite flame retardant which can be produced without using any toxic metal such as antimony trioxide or any halogen such as bromine and chlorine and can exhibit excellent flame retardancy compared with magnesium hydroxide when added in a smaller amount; and a flame retardant resin composition having excellent flame retardancy and excellent physical properties. The present invention relates to a flame retardant including: (A) 100 parts by mass of a reaction product of piperazine with one phosphorus compound selected from phosphoric acid, pyrophosphoric acid, and polyphosphoric acid; (B) 10 to 1000 parts by mass of a reaction product of melamine with a polyacid selected from cyanuric acid, phosphoric acid, pyrophosphoric acid, and polyphosphoric acid; and (C) 0.1 to 100 parts by mass of a reaction product of calcium or magnesium with silicic acid.