A liquid treatment device that achieves an effective treatment by forcibly contacting a swirl flow of a hydrogen peroxide-containing treated liquid with a liquid to be treated that has been processed to contain copper ions or iron ions includes a rod-like first electrode, a plate-like second electrode formed of a copper- or iron-containing metal, and a first treatment vessel that causes a liquid to swirl and generate a gas phase in a swirl flow of the liquid. A pulse voltage is applied to the generated gas phase to generate a plasma. The second electrode serves as an anode, and reaches inside of a supply section for a liquid to be treated. The liquid to be treated containing the generated copper ions or iron ions is forcibly brought into contact with hydrogen peroxide generated by the plasma. In this way, the Fenton's reaction effectively takes place, and the liquid treatment performance improves.