A phosphorus flame-retardant composition characterized in that it comprises an aromatic diphosphate compound represented by the general formula (I): wherein R1 and R2 are, the same or different, a lower alkyl group, R3 and R4 are, the same or different, a hydrogen atom or a lower alkyl group, Y is a bonding arm, a -CH2-, -C(CH3)2-, - S-, -S02-, -O-, -CO- or -N=N- group, k is 0 or 1, and m is an integer from 0 to 4, and it contains, as an impurity, a phosphorus compound having a hydroxyphenyl group represented by the general formula (II): wherein R1, R2, R3, R4, Y, k and m are as defined in the general formula (I), the content of the phosphorus compound being 1% by area or less as determined by gel permeation chromatography (GPC).