A flame retardant aid which serves to provide synthetic resins with sufficient flame retardancy when used together with a decreased amount of aluminum or magnesium hydroxide, and serves to give resin compositions which have color stability against heat and can be almost freely colored. The flame retardant aid is at least one compound selected from the group consisting of:a composite metal compound of the formula (A),Mg.sub.1-(x+y) Ni.sub.x Co.sub.y (OH).sub.2 (A)ora composite metal compound of the formula (B),Mg.sub.1-(x+y) Ni.sub.x Co.sub.y O (B)wherein x and y are defined in the specification,nickel halides,ammonium-containing salts of nickel,inorganic acid salts of nickel, andfatty acid salts of nickel.