To provide a method for treating a waste liquid to remove copper ions from the waste liquid.

The method for treating the waste liquid is used for treating the waste liquid containing the copper ions, and includes the following steps of: adjusting the pH value of the waste liquid; adding a precipitant and reacting it with the waste liquid to form a solution of a copper compound 41; and subjecting the waste liquid to a separation operation of the copper compound 41 to separate the waste liquid into the copper compound 41 and a first separation liquid 34. Thus, the copper ions are substantially removed from the waste liquid, and the waste liquid after the removal of the copper ions can be recovered to be reused.