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Decomposition process of volatile organic compound dissolved into water by discharge plasma inside bubble is investigated. The experiment is conducted on decomposition of dichloromethane (DCM). Argon or oxygen gas is injected into the water through a vertically positioned glass tube, in which high-voltage wire electrode is placed to generate plasmas at low applied voltage. The nanosecond pulsed high voltage generated by an inductive energy storage system pulsed power generator is applied to the tungsten wire to generate streamer discharge. Total organic carbon decreases and chloride ion concentration increases by the discharge treatment. To clarify the decomposition process, DCM and indigo carmine (IC) mixed solution is employed as specimen. The amount of IC decomposition decreases with increasing the DCM concentration. On the other hands, the amount of DCM decomposition is almost independent of IC concentration. These results show that DCM is quickly volatilized into the bubble, which is decomposed by hydroxyl radical produced by the discharge in gas phase inside bubble.