Abstract: In the coexistence of an a 5.about.20 molar excess of water based upon 2-phenylphenol compound which is reacted with phosphorus trihalide to form a 6-halo-(6H)-dibenzo[c.e][1,2]-oxaphosphorin compound and an organic solvent which is unsoluble in water and is inert in the reaction system, an organic phosphorus compound expressed by the structural formula (1) is hydrolyzed under heating to obtain a biphenyl phosphorus compound expressed by the general formula (2) and then, a cyclising dehydration reaction under heating of the resulting compound is carried out to obtain an organic phosphorus compound expressed by the structural formula (3). This constitution can produce a product of high purity and high quality without using specific chemicals or an equipment made of specific materials.