A chemical decontamination method includes a dissolution step in which a radioactive insoluble substance containing a metal oxide, the radioactive insoluble substance being adhered to a decontamination object including carbon steel, is dissolved in a decontamination solution and a metal-ion removal step in which the decontamination solution containing the metal ion, the decontamination solution being produced in the dissolution step, is brought into contact with a cation-exchange resin in order to remove the metal ion, the dissolution step including a reductive dissolution step conducted using a decontamination solution containing formic acid, ascorbic acid and/or erythorbic acid, and a corrosion inhibitor.