PURPOSE: To obtain the titled compound useful as a synthetic intermediate for an organophosphorus compound as a raw material for flame-retardant and heat-resistant polyesters, by heating a specific organo-phosphorus compound in the presence of a specific solvent and removing a produced water from the reaction system by azeotropic distillation.

CONSTITUTION: An organophosphorus compound expressed by formula I (n≥3; H of alkylene chain may be substituted) is heated preferably at 100W130°C for 4W18hr in the presence a solvent capable of being azeotropically distilled with water. The water generated by cyclization reaction is removed from the reaction system by azeotropic distillation with the solvent to afford the aimed compound expressed by formula II. The solvent is preferably used in an amount of 3W5 times of the organophosphorus compound expressed by formula I.