Provided is a novel phosphorus-containing compound represented by general formula (I). The compound has reactivity with a glycidyl group and is therefore capable of providing a curing epoxy resin composition that is expected to achieve flame retardation and reduction of dielectric constant. wherein m is a number of 1 to 10; R1, R2, R3, and R4 are each hydrogen, alkyl, or aryl; R5 is alkyl, alkanediyl, alkanetriyl, alkanetetrayl, or an aromatic group; X is oxygen or sulfur; Y is oxygen, sulfur, or ═NR′; and R′ is hydrogen, alkyl, or aryl.