To prevent environmental pollution caused by the decomposition of compounds by making halogenated aromatic hydrocarbon compounds and others be contacted with functional water produced by electrolyzing water containing an electrolyte while being irradiated with light and neutralizing the obtained liquid.

Water containing electrolyte is supplied from a pipe 111 to a water tank 101. Electric power is supplied from a power source to electrodes 103, 105, and acidic water is produced on the side of the electrode 105. Halogenated aliphatic hydrocarbon compounds and aromatic compounds are supplied from a pipe 115 to the anode 105 side, irradiated with light from a light source 166 installed inside or outside the tank 101, and contacted with functional water to promote the decomposition by light irradiation. Decomposed gas is discharged from a discharge pipe 121. After that, the deactivated functional water is discharged form a discharge pipe 181 to a tank 119. In this process, the pH of the mixed aqueous solution is adjusted by a pH adjusting means 167.