PURPOSE: To obtain a polyphenol useful as a sealing material and electrical insulation material without using post-treatment and particular apparatus indispensable to the removal of acid catalyst by carrying out dehydrative condensation of a phenolic compound with an aromatic aldehyde in the presence of an acid catalyst insoluble in the reaction system.

CONSTITUTION: A polyphenol useful as an intermediate for electrical insulation material, powder coating material and epoxy resin can be produced without problems of the disposal of a large among of waste water necessary for the removal of acid catalyst and the corrosion of apparatus caused by acid catalyst, by carrying out dehydrative condensation of a phenolic compound with an aromatic aldehyde in the presence of an acid catalyst insoluble in the reaction system, e.g., an organic solid acid catalyst such as cation exchange resin, a clay mineral such as acid clay, activated clay and clarite, an inorganic solid acid catalyst such as silica alumina, silica magnesia or alumina boria or an inorganic metallic compound such as sulfate, carbonate, nitrate, sulfide, etc., of zinc, aluminum, calcium, etc.