A method of treating a waste liquid for recovering gypsum having a small amount of fluorine from an acidic waste liquid containing fluorine and a heavy metal and removing the heavy metal, the method comprising: an aluminum dissolution step of dissolving metal aluminum in the acidic waste liquid to stably dissolve fluorine in the liquid as a fluoroaluminate ion and produce a reduced heavy metal precipitate and performing separation into a first treated water and the reduced heavy metal precipitate; a gypsum recovery step of, after removing the reduced heavy metal precipitate, adding a calcium compound to the first treated water at a liquid property of a pH of 4 or less to produce gypsum, and performing separation into a second treated water and the gypsum; a heavy metal coprecipitation step of, after removing the gypsum, adding a ferric compound to the second treated water to produce a ferric hydroxide precipitate, causing the heavy metal in the liquid to be coprecipitated with the precipitate, and performing separation into a third treated water and a heavy metal coprecipitate; an aluminum and fluorine removal step of, after removing the heavy metal coprecipitate, adding an alkali to the third treated water to adjust a pH to 5.5 to 9.5 and produce a precipitate containing aluminum and fluorine while suppressing an amount of the precipitate, and performing separation into a fourth treated water and the precipitate containing aluminum and fluorine; and a neutralization step of, after removing the precipitate containing aluminum and fluorine, further adding an alkali to the fourth treated water to adjust the pH to 9.5 to 11.8 and produce a neutralized precipitate of a heavy metal hydroxide, and performing separation into an alkali neutralization treated water and the neutralized precipitate of a heavy metal hydroxide, wherein a reduction reaction occurs by dissolving the metal aluminum in the acidic waste liquid in the aluminum dissolution step, and an oxidation-reduction potential in the reduction reaction is +400 mV or less, and in the aluminum dissolution step, a molar ratio Al/F of an amount of aluminum with respect to an amount of fluorine dissolved in the liquid is 0.4 or more and 0.8 or less. an aluminum dissolution step of dissolving metal aluminum in the acidic waste liquid to stably dissolve fluorine in the liquid as a fluoroaluminate ion and produce a reduced heavy metal precipitate and performing separation into a first treated water and the reduced heavy metal precipitate; a gypsum recovery step of, after removing the reduced heavy metal precipitate, adding a calcium compound to the first treated water at a liquid property of a pH of 4 or less to produce gypsum, and performing separation into a second treated water and the gypsum; a heavy metal coprecipitation step of, after removing the gypsum, adding a ferric compound to the second treated water to produce a ferric hydroxide precipitate, causing the heavy metal in the liquid to be coprecipitated with the precipitate, and performing separation into a third treated water and a heavy metal coprecipitate; an aluminum and fluorine removal step of, after removing the heavy metal coprecipitate, adding an alkali to the third treated water to adjust a pH to 5.5 to 9.5 and produce a precipitate containing aluminum and fluorine while suppressing an amount of the precipitate, and performing separation into a fourth treated water and the precipitate containing aluminum and fluorine; and a neutralization step of, after removing the precipitate containing aluminum and fluorine, further adding an alkali to the fourth treated water to adjust the pH to 9.5 to 11.8 and produce a neutralized precipitate of a heavy metal hydroxide, and performing separation into an alkali neutralization treated water and the neutralized precipitate of a heavy metal hydroxide, wherein a reduction reaction occurs by dissolving the metal aluminum in the acidic waste liquid in the aluminum dissolution step, and an oxidation-reduction potential in the reduction reaction is +400 mV or less, and in the aluminum dissolution step, a molar ratio Al/F of an amount of aluminum with respect to an amount of fluorine dissolved in the liquid is 0.4 or more and 0.8 or less.