A flame retardant agent for a thermoplastic resin, which enables the thermoplastic resin containing the flame retardant agent to keep its viscous property, to be made into a fiber readily, and to exhibit flame retardant effect even when ammonium dihydrogen phosphate is contained in the flame retardant agent as a non-halogen flame retardant component. Also disclosed is a flame retardant resin composition containing the flame retardant agent. Specifically, the flame retardant agent for a thermoplastic resin comprises ammonium dihydrogen phosphate, potassium hydroxide, glycerin and urea, wherein the potassium hydroxide is contained in an amount of 10 to 70 parts by weight, glycerin is contained in an amount of 0.1 to 4.0 parts by weight, and urea is contained in an amount of 1 to 9 parts by weight relative to 100 parts by weight of ammonium dihydrogen phosphate.