Active ester resin containing phosphorus atom, epoxy resin composition and cured product thereof, prepreg, circuit board, and build-up film

Provided are: an active ester resin containing a phosphorus atom, which enables the provision of a cured article having all of flame retardancy, heat resistance and dielectric properties; an epoxy resin composition containing the active ester resin as a curing agent; a cured product of the epoxy resin composition; a prepreg; a circuit board; and a build-up film. An active ester resin containing a phosphorus atom, which is characterized by having such a structure (I) that a structural unit (α) produced by linking multiple aromatic cores (X) to each other through an aliphatic cyclic hydrocarbon group is linked to another structural unit (α) through an arylene dicarbonyloxy group, wherein at least one of the aromatic cores (X) in the resin has a structural moiety (Y) represented by any one of structural formulae (y1) to (y4) as a substituent on the at least one of the aromatic core.