A naphthol-modified phenolic resin having low softening point and good flowability is obtained by allowing a naphthol and a phenol to react with an aldehyde in the presence of an acid catalyst at 60 to 110.degree. C. to obtain a primary reaction mixture, heating the primary reaction mixture in the presence of a strong acid, a super acid or a mixture thereof under ambient pressure at 120 to 180.degree. C. for 1 to 12 hours to obtain a secondary reaction mixture, and subjecting the secondary reaction mixture to vacuum concentration or steam distillation, and the naphthol-modified phenolic resin is suited to use as a curing agent in epoxy resin molding materials for sealing electronic parts.