PURPOSE: To prepare the titled compound useful as a flame-retardant, a resin additive, etc., by an easily controllable reaction process which comprises the reaction of a specific compound with a compound obtained by the reaction of a diamino-cyclic phsphazene compound and an organic isocyanate.

CONSTITUTION: A diamino-cyclic phosphazene compound of formula I (R is H or lower alkyl; X is halogen; n is 3 or 4) is made to react with a compound of formula II (R1 is alkylene, alkyl-substituted aromatic group residue, etc.) to obtain a ureylene-cyclic phosphazene compound of formula III (m is 1W8). The reaction is carried out in an organic solvent at 50W130°C. The objective phsophorus-containing oligomer of formula V can be prepared by reacting the compound of formula III with a compound of formula IV (R2 is alkoxy, alkyl, etc.; Y is H or alkali metal) at 0W 140°C.