To surely make the amount of hexavalent chromium detected from waste water and sludge a standard value or lower without drastically changing generally installed waste water treatment facilities.

In this method, in treating a waste liquid and waste water for chromate treatment used for zinc and zinc alloy plating not containing hexavalent chromium but cobalt, one or more kinds of sodium sulfide, potassium sulfide, ammonium sulfide, ammonium hydrosulfide, sodium hydrosulfide, potassium hydrosulfide, ammonium polysulfide, sodium polysulfide, and potassium polysulfide to sulfide such as waste liquid and waste water.