A method for producing a semiconductor treatment liquid used for treating a resin having an ether bond, the method comprising: heating a phosphoric acid-containing solution to 100 C. or higher and 400 C. or lower to produce a heated solution; cooling the heated solution to 5 C. or higher and 95 C. or lower to produce a cooled solution; and mixing the cooled solution with at least one selected from the group consisting of an aqueous hydrogen peroxide solution and an aqueous nitric acid solution.