PURPOSE: To obtain a flame retardant, having excellent flame retardant performance without cracking in a resin even by being added to the resin and emitting gases in processing and suitable for polyesters, etc., by reacting a compound, such as resorcin, with phosphorus oxychloride and reacting the resultant product with phenol, etc.

CONSTITUTION: A compound selected from resorcin, hydroquinone, bisphenol A, bisphenol sulfone and bisphenolmethane is reacted with phosphorus oxychloride and the unreacted phosphorus oxychloride is then removed under reduced pressure condition. The obtained product is subsequently reacted with one or more of phenol, cresol and xylenol to afford the aimed high polymer phosphorus compound flame retardant, expressed by formula I [R1WR4 are phenyl, cresol, etc.; R5 is formula II or III (A is SO2, CH2, etc.) is 1W20] with ≤2wt.% content of low-molecular phosphorus compound triaryl phosphate.