A water treatment device generates a bubble in water to be treated inside a treatment tank, and treats the water to be treated through use of radicals that are generated by bubble electric discharge caused via the bubble. The water treatment device includes main electrodes for forming a main discharge area between a first main electrode and a second main electrode, auxiliary electrodes for forming a preliminary discharge area between a first auxiliary electrode and a second auxiliary electrode, and a bubble generator generating a bubble in water to be treated through use of an externally supplied gas. When a bubble generated by the bubble generator passes through the preliminary discharge area, discharge is caused via the bubble, and, when the bubble in an excited state subsequently passes through the main discharge area, the bubble causes discharge again to generate radicals.