Water treatment device and water treatment method

A water treatment device is provided with: a ground electrode (2) extending in the axial direction; a high-voltage electrode (5) disposed coaxially so as to surround the ground electrode (2) from the outside in the radial direction; a dielectric (1) that forms an annular gap between the ground electrode (2) and the dielectric (1) and generates dielectric barrier discharge; and a water film-forming section (7) in which an annular flow path (7c) that opens toward the annular gap is formed between the water film-forming section (7) and the ground electrode (2) along the circumferential direction at an interval narrower than the annular gap on one end side in the axial direction, and in which the annular flow path (7c) opens toward the annular gap when the one end side is oriented upward and the axial direction is set to be vertical. The water to be treated (90) flows down toward the other end as a water film (91) covering the ground electrode (2). 水处理装置具备：接地电极(2)，其沿轴向延伸；高压电极(5)，其以从径向的外侧将接地电极(2)包围的方式同轴配置；电介质(1)，其