To provide a method for making it possible not only to almost perfectly kill water bloom but also to suppress the growth of water bloom in a closed water area by a simple apparatus constitution, and an apparatus therefor.

In this method, ozone is added to and mixed with water to be treated containing water bloom occurring/living in the closed water area such as lakes and marshes or the like in which domestic wastewater or rainwater containing fertilizer or the like of a plant flows and hydrogen peroxide is subsequently added to and mixed with the water to be treated while irradiating the water to be treated with ultrasonic waves. It is also preferable to irradiate the water to be treated with ultraviolet rays along with the irradiation with ultrasonic waves. The algicidal/growth-suppressing effect of water bloom is increased by the cavitation effect due to the irradiation of the water to be treated with ultrasonic waves.