Method for producing cyanuric acid-modified phosphorus-containing epoxy resin, resin composition containing cyanuric acid-modified phosphorus-containing epoxy resin, and cured product of same

Provided is a method for producing a cyanuric acid-modified phosphorus-containing epoxy resin, which enables the production of a flame-retardant cyanuric acid-modified phosphorus-containing epoxy resin in a short time with an extremely small amount of residual cyanuric acid, said cyanuric acid-modified phosphorus-containing epoxy resin being obtained by modifying an epoxy resin with use of cyanuric acid and a phosphorus compound and being capable of providing an epoxy resin cured product having excellent physical properties. A method for producing a cyanuric acid-modified phosphorus-containing epoxy resin that is obtained by reacting a phosphorus compound, cyanuric acid and an epoxy resin as essential components, wherein a specific phosphorus compound is in an amount of 2.5-50 moles relative to 1 mole of cyanuric acid; a resin composition that contains a curing agent and a cyanuric acid-modified phosphorus-containing epoxy resin which is obtained by the above-described method and which has a phosphorus content of 1.0-5.0% by mass and a nitrogen content of 0.1-2.0% by mass, with the total of the phosphorus content and the nitrogen content being 2.5-5.5% by mass; and a cured product of the resin composition.