A liquid treatment unit includes: a treatment tank provided with an inlet and an outlet, the treatment tank having a shape that allows a portion of liquid to be retained; a controller that controls supply of liquid into the treatment tank and ejection of liquid from the treatment tank; and a plasma generator generates plasma in the liquid inside the treatment tank. The plasma generator generates plasma while the controller stops supply of liquid and ejection of liquid. After the liquid has been treated, the controller resumes the supply of liquid, and causes the liquid to be ejected from the treatment tank while allowing a portion of the treated liquid to be retained inside the treatment tank.