PROBLEM TO BE SOLVED: To provide a curable resin composition whose cured product has both excellent heat resistance and excellent dielectric characteristics, a cured product thereof, a prepreg, a circuit board, a semiconductor sealing material, a build-up film, and a cyanate ester resin which gives these properties.SOLUTION: The cyanate ester resin contains a trifunctional compound (x) represented by the structural formula (1) (wherein Rand Reach independently represents a hydrogen atom, a C1-C4 alkyl group, or a C1-C4 alkoxy group).