The present invention provides an active ester resin containing a phosphorus atom capable of providing a cured product having all of flame retardancy, heat resistance, and dielectric characteristics, an epoxy resin composition containing the same as a curing agent, a cured product thereof, a prepreg, a circuit board, and a build-up film. The active ester resin containing a phosphorus atom has Structure (I) in which a structural unit (α) in which a plurality of aromatic nuclei (X) are connected through an alicyclic hydrocarbon group is connected with another structural unit (α) through an arylene dicarbonyloxy group, and, at least one of the aromatic nuclei (X) present in the resin has a structural site (Y) represented by any one of the following Structural Formulas (y1) to (y4) as a substituent on the aromatic nucleus.