To obtain a compound useful as a flame retardant by selecting a phosphazene derivative having a specific structure.

A phosphazene derivative represented as formula I is selected. In formula I, Q shows a bond represented as formula II wherein P is bonded to N in formula I, and N is bonded to P in formula I; n is 1-2; Z1-Z4 are each a phenyl may have a substituent without an alkyl or halogen, and naphthyl may have a substituent without a halogen. The phosphazene derivative can be prepared by reacting the halogenated phosphazene derivative represented as formula III with a compound containing a hydroxyl group represented as the formula: Z-OH (wherein Z is Z') in an organic solvent in the presence of a base. In formula III, Y shows a bond represented by formula IV, X is Cl or Br, P in Y is bonded to N in formula III, N in Y is bonded to P in formula III; and n is 1 or 2. An organic polymer composition containing this compound never generates a halogenous gas at combustion.