To provide a water quality improving device in closed water areas which improves water quality in the closed water areas such as a pond and a water reservoir.

A float 2 is mounted under a base plate 1 and a motor 4 is mounted on the base plate 1. A motor drive circuit 5 is mounted at the side of the motor 4. An ultrasonic drive circuit 6 and a high-voltage generating circuit 7 are mounted on the base plate 1, and an ultrasonic piezoelectric transducer 9 is mounted to an upper water pipe 8 under the base plate 1. A partition plate 11 is supported under the upper water pipe 8, and a lower water pipe 14 installed with an electrode 13 at the center of the lower side of the partition plate 11 is supported by the partition plate 11. An upper propeller 16 and a lower propeller 17 fixed to the drive shaft 4a of the motor 4 are mounted across a partition plate 10.

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