A new 1,3-bis(3-formyl-4-hydroxyphenyl)adamantane, which provides a material offering excellent properties such as heat resistance and mechanical strength for use as an intermediate material for adamantanebisphenol derivatives or use in photosensitive resist materials, epoxy resins and other synthetic resins, thermosensitive recording materials, and the like, can be obtained through an industrial process in an easy manner at a good yield and high purity by producing a Schiff base from a 1,3-bis(4-hydroxyphenyl)adamantane by causing it to react with a hexamethylenetetramine or other substance in the presence of an acid, and then hydrolyzing the obtained Schiff base using an acid. A new polynuclear polyphenol is also provided that may be derived from the same.