To provide a method for producing a colorless transparent phosphorus-containing resin having excellent optical characteristics, having high toughness, and having excellent heat stability and moldability at the same time.

This method for producing a phosphorus-containing resin which contains carbonate residues, phosphonic acid residues, and dihydric phenol residues comprises a first stage for acting a monomer which gives the phosphonic acid residue on a mixture containing a carbonate polymer of a dihydric phenol and the dihydric phenol in a specific ratio and a second stage for further acting the monomer which gives the phosphonic acid residue and is not used in the first stage on a reaction mixture given by the first stage, wherein each of the phosphonic acid residues is expressed by general formula (1) and each of the dihydric phenol residues is expressed by general formula (2).