To provide a one-pack epoxy resin composition which provides easy process control, little contamination and high product quality, and to provide the methods for producing an insulating coil and a cured fiber structure using the composition.

The one-pack epoxy resin composition comprises (A) a bisphenol A type and/or bisphenol F type epoxy resin which is liquid at ordinary temperature, (B) methyl hymic anhydride as an alicyclic acid anhydride-based curing agent which is liquid at ordinary temperature, (C) an imidazole-based microcapsule type accelerator as a latent N-containing accelerator, and (D) a thixotropy-imparting agent, wherein the component (A), the component (B), and the component (D) satisfy the following relation: 6.0(specific surface area [m2/g] by the BET adsorption method of component (D))*(loading [g] of component (D))/(loadings [g] of component (A) and component (B))30; and the ratio R (0.01/100) of to the viscosity in the case of a shear rate of 0.01 s-1 to the viscosity in the case of a shear rate of 100 s-1 is 2.0R(0.01/100)3.5.