The invention provides a flame-retardant processing agent capable of imparting durable flame retardance to polyester-based fiber products without using halogen-based flame retardant. The flame-retardant processing agent is obtained by dispersing at least one phosphoric acid amide selected from the group consisting of 1,4-piperazinediyl bis(diarylphosphate), diaryl aminophosphate and aryl diaminophosphate as a flame retardant in a solvent in the presence of a nonionic surfactant or an anionic surfactant.