To precipitate a plankton by inactivating that by a method wherein a phytoplankton of a water bloom, red water or the like which are generated by eutrophiction in river, sea area, lakes, ponds, etc., are irradiated with ultrasonic waves of a specific frequency in water.

When plankton cells of water bloom, red water or the like are irradiated with ultrasonic waves, the cells are inactivated, photosynthesis capacity is impeded, and they come to be easily precipitated. By executing such the precipitation treatment, the water bloom, red water, etc., are attempted to be excluded. A boat 2 is floated on a pond 1 in which the water bloom is generated when executed, and an inside of water 3 is irradiated with ultrasonic waves of 20 kiloherz or over frequency from a ultrasonic generator at suitable sounding intervals. An irradiation time at that time is determined according to a generation state of the water bloom. Further, excepting irradiation with ultrasonic waves under a state of floating the ship on the pond, water of the pond is circulated, the water under this circulation process is irradiated with ultrasonic waves, and defects are preferably attempted to be excluded thereby.