To provide a method of reducing a load of neutralization treatment of a carbonate decomposition waste liquid, improving a work environment and also capable of extracting to recover phenol from the carbonate decomposition waste liquid.

The method for treating this waste liquid comprises loading the carbonate decomposition waste liquid, generated when a raw material liquid is subjected to carbonate decomposition to separate phenol, with sulfuric acid, thereby neutralizing sodium carbonate dissolved in the carbonate decomposition waste liquid, extracting phenol remaining in the carbonate decomposition waste liquid, and carrying out waste water treatment by making the carbonate decomposition waste liquid as a sodium sulfate aqueous solution.