Abstract: It is an object to provide a method for killing aquatic organisms in liquid by which aquatic organisms in liquid can be killed at low cost, and the object is attained by a method including a first step of generating microbubbles in a range of 4 to 100 ?m from bubbles containing ozone discharged into liquid in a pipe, a second step of forcibly crushing the microbubbles in the pipe generated at the first step to generate OH radicals as a result of crushing of the microbubbles themselves due to the forcible crushing, and a third step of generating OH radicals as a result of dissolution and decomposition of the ozone contained in the microbubbles in the liquid due to the forcible crushing of the microbubbles, wherein the OH radicals generated at the second and third steps, OH radicals generated as a result of self-crushing of the microbubbles themselves in the liquid in the pipe, and OH radicals generated as a result of dissolution and decomposition of the ozone contained in the microbubbles in the liquid due to