PURPOSE: To obtain the titled polyoxadiazole with excellent heat resistance and flame retardancy, suitable for a heat resistant coating material, a building material, etc., by the dehydrating-ring closure of a polyhydrazide consisting of a repeating unit containing a phenoxaphosphine ring.

CONSTITUTION: A polyoxadiazole (poly-1,3,4-oxadiazole) having a phenoxaphosphine ring consisting of a repeating unit of formula III, is obtained by the dehydrating-ring closure of a polyhydrazide consisting of a repeating unit shown by the formula I (wherein R is a 4W8C alkylene, phenylene or formula II), in a polyphosphoric acid, at about 140W200°C. The polyhydrazide of the formula I is obtained by the polycondensation of a phenoxaphosphine ring compound of the formula IV (wherein Y is a halogen or hydroxy group) with a dihydrazide of the formula V.