To provide a method for stably producing a prepreg on an industrial scale while improving the storage stability of an organic solvent solution of a biphenylaralkyl novolac epoxy resin.

The method for the production of a prepreg comprises the impregnation or coating of a substrate with a thermosetting resin composition varnish containing an organic solvent and a prepolymer of a biphenylaralkyl novolac epoxy resin obtained by heating a biphenylaralkyl novolac epoxy resin in the presence or absence of a catalyst, and the drying of the substrate impregnated or coated with the varnish.