Abstract: A regional water purifier collects blue green algae continuously by using a water flow produced by the operation of a water flow generator, and continuously and efficiently processes the algae to make it inactive by irradiating ultrasonic waves thereon. Thus, regional water purification is performed efficiently at a far lower cost than the conventional method through the blue green algae processing. The purifier includes a blue green algae processing duct 2, which is disposed under water where blue green algae breed in large quantities, and has an ultrasonic wave transmitter 3 therein for transmitting ultrasonic waves to process the blue green algae with the ultrasonic waves. Both ends of the duct 2 are open, and a water flow generator 4 produces a suction flow for sucking water containing blue green algae into the blue green algae. processing duct 2 from a first end 2a and a discharge flow for discharging the water from the second end 2b.