An organic phosphorus-based compound which satisfies the following requirements (i) to (viii) and is represented by the following formula : (i) the organic purity should be not lower than 98 %; (ii) the solubility in 20°C water should be not higher than 0.1 g/100 g of water; (iii) the total halogen component content should be not higher than 1,000 ppm; (iv) the total volatile organic matter content should be not higher than 800 ppm; (v) the Δ pH value should be not larger than 1.0; (vi) the volume standard median diameter should be not larger than 30 μm; (vii) the maximum particle diameter should be not larger than 200 μm; and (viii) the standard deviation represented by the following formula should be not larger than 20 μm. Standard deviation = d 84 % + d 16 % / 2 d84 %: particle diameter at a point where the cumulative curve becomes 84 % (μm) d16 %: particle diameter at a point where the cumulative curve becomes 16 % (μm) In the above formula, R 2 and R 5 may be the same or different and are each a phenyl group which may have a substituent, naphthyl group which may have a substituent, anthryl group which may have a substituent, or branched or linear alkyl group having 1 to 4 carbon atoms which may have an aromatic substituent. R 1 , R 3 , R 4 and R 6 may be the same or different and are each a hydrogen atom, branched or linear alkyl group having 1 to 4 carbon atoms, phenyl group which may have a substituent, naphthyl group which may have a substituent, or anthryl group which may have a substituent. (i) the organic purity should be not lower than 98 %; (ii) the solubility in 20°C water should be not higher than 0.1 g/100 g of water; (iii) the total halogen component content should be not higher than 1,000 ppm; (iv) the total volatile organic matter content should be not higher than 800 ppm; (v) the Δ pH value should be not larger than 1.0; (vi) the volume standard median diameter should be not larger than 30 μm; (vii) the maximum particle diameter should be not larger than 200 μm; and (viii) the standard deviation represented by the following formula should be not larger than 20 μm. Standard deviation = d 84 % + d 16 % / 2 d84 %: particle diameter at a point where the cumulative curve becomes 84 % (μm) d16 %: particle diameter at a point where the cumulative curve becomes 16 % (μm) In the above formula, R 2 and R 5 may be the same or different and are each a phenyl group which may have a substituent, naphthyl group which may have a substituent, anthryl group which may have a substituent, or branched or linear alkyl group having 1 to 4 carbon atoms which may have an aromatic substituent. R 1 , R 3 , R 4 and R 6 may be the same or different and are each a hydrogen atom, branched or linear alkyl group having 1 to 4 carbon atoms, phenyl group which may have a substituent, naphthyl group which may have a substituent, or anthryl group which may have a substituent. d84 %: particle diameter at a point where the cumulative curve becomes 84 % (μm) d16 %: particle diameter at a point where the cumulative curve becomes 16 % (μm) In the above formula, R 2 and R 5 may be the same or different and are each a phenyl group which may have a substituent, naphthyl group which may have a substituent, anthryl group which may have a substituent, or branched or linear alkyl group having 1 to 4 carbon atoms which may have an aromatic substituent. R 1 , R 3 , R 4 and R 6 may be the same or different and are each a hydrogen atom, branched or linear alkyl group having 1 to 4 carbon atoms, phenyl group which may have a substituent, naphthyl group which may have a substituent, or anthryl group which may have a substituent. <EOS>