An epoxy curing agent comprising a heterocycle-containing compound having a backbone clain selected from the group consisting of polyether, polyvinyl, polyester, polyamide, polycarbonate, and novolac chains and at least two heterocyclic groups of the following general formula (1) as side chains, and a one-component (type) epoxy resin composition comprising said epoxy curing agent and a polyepoxy compound, ##STR1## wherein R.sup.1 and R.sup.2 may be the same or different and each represents hydrogen, straight-chain or branched C.sub.1 to C.sub.6 alkyl or alkenyl, or C.sub.6 to C.sub.8 aryl; or R.sup.1 and R.sup.2, taken together with the adjacent carbon atom, represents C.sub.5 to C.sub.7 cycloalkyl; R.sup.3 represents C.sub.1 to C.sub.10 alkylene. The object is to provide the one-component (type) epoxy resin composition having fast-curing feature, an improved storage stability, and an improved degree of workability.