A method for decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds characterized by contacting the compound to be decomposed with functional water under light irradiation, wherein the functional water is characterized by a hydrogen ion concentration (pH) of 1-4, an oxidation-reduction potential of 800-1500 mV (working electrode: platinum, reference electrode: silver-silver chloride) and a chlorine concentration of 5-150 mg/l, and the irradiation is carried out with light of a wavelength of 300 nm or more, with an intensity of 10 mW/cm2 or less. This method is carried out at room temperature under the atmospheric pressure. An apparatus for the method is also provided.