methylol compound represented by the formula [22]: ##STR29## (wherein R28 is a hydrogen atom, a methyl group or an ethyl group) in a solvent such as water, diethylene glycol, glycerin or the like, or a mixture of such solvents, or without solvent, in the presence of an acid (such as oxalic acid, sulfuric acid, phosphoric acid, p-toluenesulfonic acid or the like) at 50-180??? for 0.5-20 hours with