PROBLEM TO BE SOLVED: To provide a treatment system materializing reduction in a supplementary water amount, maintaining of solvent absorption efficiency, high concentration of a volatile organic compound of treated liquid to be discharged, and reduction in an installation space.SOLUTION: There is provided a volatile organic compound reduction device reducing volatile organic compound using gas-liquid contact process, which brings gas to be treated containing an aqueous volatile organic compound passing through a single treatment tower into contact with treatment liquid to be supplied to the treatment tower, and which makes the treatment liquid absorb the volatile organic compound in the gas to be treated. The volatile organic compound reduction device has: a treatment tower 1 in which the gas to be treated is introduced; two circulation tanks 2A, 2B arranged in parallel with each other which supply the treatment liquid to the treatment tower; switching means 8, 20 of the circulation tanks for supplying the treatment liquid from one of the circulation tanks to the treatment tower and returning the treatment liquid from the treatment tower; means 25 for controlling and reducing concentration of the volatile organic compound in the treatment gas by managing the concentration of the volatile organic compound in the treatment liquid equal to or less than a set value; and draining means 23A, 23B which are provided in the two circulation tanks respectively and selectively drain the treatment liquid.
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