Process for preparing a phosphorus flame-retardant composition, wherein said composition comprises: an aromatic diphosphate compound represented by the general formula: wherein R 1 and R 2 are, the same or different, a lower alkyl group, R 3 and R 4 are, the same or different, a hydrogen atom or a lower alkyl group, Y is a bonding arm, a-CH 2 -, -C(CH 3) 2 -, -S-, -SO 2 -, -O-, -CO- or -N=N- group, k is 0 or 1, and m is an integer from 0 to 4, and, as an impurity, a phosphorus compound having a hydroxyphenyl group represented by the general formula: wherein R 1, R 2, R 3, R 4, Y, k and m are as defined in the general formula, the content of the phosphorus compound being 1% by area or less as determined by gel permeation chromatography, characterized in that said process comprises the following steps: - Step 1, wherein an aromatic monohydroxy compound having a group for giving steric hindrance at the ortho position represented by the general formula: wherein R 1, R 2 and R 3 are as defined in the general formula, reacts with a phosphorus oxyhalide in the presence of a Lewis acid catalyst to obtain a diaryl phosphorohalidate represented by the general formula: wherein R 1, R 2 and R 3 are as defined in the general formula, and X represents a halogen, and optionally in the presence of a solvent, wherein after completion of the reaction the optional solvent, unreacted aromatic monohydroxy compound and phosphorus oxyhalide are removed under a reduced pressure of 30 kPa or less; - Step 2, which is carried out successively in the same reaction vessel as Step 1, without going through purification of the diaryl phosphorohalidate obtained in Step 1, wherein the diaryl phosphorohalidate obtained in Step 1 reacts with an aromatic dihydroxy compound that is stoichiometrically equivalent to the amount of the diaryl phosphorohalidate calculated from the amount of the reaction mixture obtained in Step 1 and from the halogen concentration in the reaction mixture, said aromatic dihydroxy compound being represented by the general formula: wherein R 4, Y, k and m are as defined in the general formula in the presence of a Lewis acid catalyst. an aromatic diphosphate compound represented by the general formula: wherein R 1 and R 2 are, the same or different, a lower alkyl group, R 3 and R 4 are, the same or different, a hydrogen atom or a lower alkyl group, Y is a bonding arm, a-CH 2 -, -C(CH 3) 2 -, -S-, -SO 2 -, -O-, -CO- or -N=N- group, k is 0 or 1, and m is an integer from 0 to 4, and, as an impurity, a phosphorus compound having a hydroxyphenyl group represented by the general formula: wherein R 1, R 2, R 3, R 4, Y, k and m are as defined in the general formula, the content of the phosphorus compound being 1% by area or less as determined by gel permeation chromatography, characterized in that said process comprises the following steps: - Step 1, wherein an aromatic monohydroxy compound having a group for giving steric hindrance at the ortho position represented by the general formula: wherein R 1, R 2 and R 3 are as defined in the general formula, reacts with a phosphorus oxyhalide in the presence of a Lewis acid catalyst to obtain a diaryl phosphorohalidate represented by the general formula: wherein R 1, R 2 and R 3 are as defined in the general formula, and X represents a halogen, and optionally in the presence of a solvent, wherein after completion of the reaction the optional solvent, unreacted aromatic monohydroxy compound and phosphorus oxyhalide are removed under a reduced pressure of 30 kPa or less; - Step 2, which is carried out successively in the same reaction vessel as Step 1, without going through purification of the diaryl phosphorohalidate obtained in Step 1, wherein the diaryl phosphorohalidate obtained in Step 1 reacts with an aromatic dihydroxy compound that is stoichiometrically equivalent to the amount of the diaryl phosphorohalidate calculated from the amount of the reaction mixture obtained in Step 1 and from the halogen concentration in the reaction mixture, said aromatic dihydroxy compound being represented by the general formula: wherein R 4, Y, k and m are as defined in the general formula in the presence of a Lewis acid catalyst. - Step 1, wherein an aromatic monohydroxy compound having a group for giving steric hindrance at the ortho position represented by the general formula: wherein R 1, R 2 and R 3 are as defined in the general formula, reacts with a phosphorus oxyhalide in the presence of a Lewis acid catalyst to obtain a diaryl phosphorohalidate represented by the general formula: wherein R 1, R 2 and R 3 are as defined in the general formula, and X represents a halogen, and optionally in the presence of a solvent, wherein after completion of the reaction the optional solvent, unreacted aromatic monohydroxy compound and phosphorus oxyhalide are removed under a reduced pressure of 30 kPa or less; - Step 2, which is carried out successively in the same reaction vessel as Step 1, without going through purification of the diaryl phosphorohalidate obtained in Step 1, wherein the diaryl phosphorohalidate obtained in Step 1 reacts with an aromatic dihydroxy compound that is stoichiometrically equivalent to the amount of the diaryl phosphorohalidate calculated from the amount of the reaction mixture obtained in Step 1 and from the halogen concentration in the reaction mixture, said aromatic dihydroxy compound being represented by the general formula: wherein R 4, Y, k and m are as defined in the general formula in the presence of a Lewis acid catalyst. wherein R 4, Y, k and m are as defined in the general formula in the presence of a Lewis acid catalyst.