An epoxy-resin composition comprising (A) an at least bifunctional epoxy compound and/or an at least bifunctional epoxy resin, (B) an at least bifunctional ester-containing compound and/or an at least bifunctional ester-containing resin as a curing agent, 10 to 100 % of whose hydroxy groups are esterified with aliphatic or aromatic acyl groups, and (C) an accelerating agent, the accelerating agent essentially containing a phosphine oxide represented by general formula (1):    where R 1 to R 6 all of which may be the same or not are hydrogen, straight, branched or cyclic alkyl having 1 to 10 carbons or aryl or aralkyl having 6 to 10 carbons.