PURPOSE: To produce the titled compound useful as an additive for imparting fibers with flame-resistance and fire-retardance, easily in an industrial scale, by heating a partially alkoxylated phosphonitrile chloride in the presence of a quaternary ammonium salt.

CONSTITUTION: A phosphonitrile chloride partially alkoxylated with an alcohol or a metal alkoxide (the carbon numbers of both compounds are preferably 1W7) (preferably a cyclic phosphazene oligomer rich in trimer and tetramer, and having a residual active chlorine concentration of 1W16wt% after the partial alkoxylation) is heated at 40W180°C under reduced pressure of usually 5W350mmHg in the presence of a quaternary ammonium salt. The amount of the quaternary ammonium salt is preferably 1W1.5 equivalent based on the residual active chlorine of the partially alkoxylated compound.