The present invention relates to a phosphorus-containing epoxy resin, which is an epoxy resin modified with a side chain having a reactive phosphorus-containing compound. Also, the present invention relates to a flame-retardant resin composition, which comprises: (1) the phosphorus-containing epoxy resin, (2) a halogen-free hardener having a reactive hydrogen capable of reacting with the epoxy group in epoxy resin, and (3) a hardener promoter. The flame-retardant resin composition described above has improved excellent heat resistance and flame retardant property, and is especially suitable for adhesive laminates, composite materials, printed circuit boards, adhesive material for copper foils, and is suitable for IC packaging materials.