A method for treating an isocyanate residue, which comprises carrying out a thermal decomposition reaction of a carbamate that is produced by the reaction among an amine, urea and/or an N-unsubstituted carbamic acid ester and an alcohol to produce a decomposition solution, separating an isocyanate and the alcohol from the decomposition solution to produce the isocyanate residue, and bringing the isocyanate residue into contact with high-pressure/high-temperature water to decompose the isocyanate residue into an amine; and a method for treating a carbonate, which comprises bringing the carbonate into contact with high-pressure/high-temperature water to decompose the carbonate into an alcohol.