Provided are a novel resin and a precursor thereof, which have excellent pyrolytic properties and solubility in solvents. Said resin is obtained by subjecting a mono alkyl naphthalene formaldehyde resin, obtained by reacting a compound represented by formula (1) and formaldehyde in the presence of catalyst, to acidic treatment. In formula (1), R1 represents an alkyl group with a carbon number of one to four.